Wechsel im Kontext: blank the article, make the verb decide, add the rule

The level showed the article already in the sentence ("unter dem Stuhl"),
so it was answerable by recognising "dem" — it never tested wohin/wo at
all. And the frame pool was tiny: one cat, one lamp, eight nouns.

- The article is now blanked. The only evidence is the verb and whether it
  has a direct object, which is the actual skill. The solved sentence is
  revealed on answer, contracted where German really contracts (ins Regal,
  am Bahnhof), with a note naming the fusion.
- Frames rebuilt around what decides the case: 27 new nouns, 18 new verbs,
  and every frame one-way so no verb+noun pairing licenses both readings.
- Motion verbs now appear on the DATIVE side on purpose — schwimmen im See,
  tanzen in der Kueche, spielen auf der Strasse — because "is it moving?" is
  the wrong test and that is the main thing learners get wrong.
- The put/be pairs are drilled properly: legen/liegen, stellen/stehen,
  setzen/sitzen, plus haengen and stecken, which keep the same present tense
  either way so only the direct object tells you.
- ankommen added as the lexical exception: Dativ despite obvious arrival.
- New "Wohin oder wo?" info panel: the rule, the before-and-after test, the
  movement trap, the verb-pair table and the exception. Reachable from the
  practice screen, from the answer feedback, and from Charts.
- Feedback now explains the specific sentence rather than restating theory.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Uza6mhTpAvSwMQuFc4Cz8

src/App.jsx

@@ -183,6 +183,7 @@ export default function App() {
   const [ok, setOk] = useState(false);
   const [res, setRes] = useState([]);
   const [chart, setChart] = useState(false);
+  const [rule, setRule] = useState(false);
   const [learnStep, setLearnStep] = useState(0);
   const [st, setSt] = useState({sc:{},tc:0,ta:0,streak:0,ld:null});
   const [access, setAccess] = useState(null);
@@ -283,6 +284,7 @@ export default function App() {
 
   useEffect(() => {
     const h = e => {
+      if (rule) { if (e.key === 'Escape') setRule(false); return; }
       if (chart) { if (e.key === 'Escape') setChart(false); return; }
       if (scr === 'summary' && e.key === 'Enter') { e.preventDefault(); summaryRef.current?.(); return; }
       if (scr !== 'practice') return;
@@ -640,30 +642,36 @@ export default function App() {
 
         {/* PRACTICE: wechsel in context */}
         {scr === 'practice' && q?.type === 'wechselContext' && (
-          <div style={{textAlign:'center', paddingTop:18}}>
-            <div style={{...S.sentence, animation:shake?'shake 0.3s':'none'}}>{q.sentence}</div>
-            {(!hintHidden('translation') || rev) ? (
-              <>
-                <div style={S.translation}>
-                  {q.translation}
-                  {!rev && <button style={{...S.hintToggle, marginLeft:8}} onClick={()=>toggleHint('translation')}>hide</button>}
-                </div>
-                <div style={S.keyInfo}>
-                  <span style={{fontWeight:700}}>{q.prepKey}</span>
-                  <span style={S.keyInfoMute}> ({PREPS[q.prepKey].en})</span>
-                </div>
-              </>
-            ) : (
-              <>
-                <div style={S.translation}>
-                  <button style={S.hintToggle} onClick={()=>toggleHint('translation')}>show translation</button>
-                </div>
-                <div style={S.keyInfo}>
-                  <span style={{fontWeight:700}}>{q.prepKey}</span>
-                </div>
-              </>
-            )}
-            <div style={S.secondaryContext}>Wechselpräposition · motion or location?</div>
+          <div style={{textAlign:'center', paddingTop:10}}>
+            <div style={{...S.sentence, animation:shake?'shake 0.3s':'none'}}>
+              {q.sentence.split('_____').map((part,i,a)=>(
+                <span key={i}>
+                  {part}
+                  {i < a.length-1 && (
+                    <span style={{...S.blank,
+                      borderBottomColor: rev?(ok?GREEN:RED):'#bbb',
+                      color: rev?(ok?GREEN:RED):'#1a1a1a',
+                    }}>
+                      {rev ? q.article : '     '}
+                    </span>
+                  )}
+                </span>
+              ))}
+            </div>
+
+            <Hideable hidden={hintHidden('translation') && !rev} onToggle={()=>toggleHint('translation')} label="translation" style={S.translation}>
+              {q.translation}
+            </Hideable>
+
+            <Hideable hidden={hintHidden('prepHint')} onToggle={()=>toggleHint('prepHint')} label="preposition" style={S.keyInfo}>
+              <span style={{fontWeight:700}}>{q.prepKey}</span>
+              <span style={S.keyInfoMute}> ({PREPS[q.prepKey].en})</span>
+            </Hideable>
+
+            <div style={S.secondaryContext}>
+              Wechselpräposition · wohin or wo?
+              <button style={S.ruleBtn} onClick={()=>setRule(true)}>ⓘ Rule</button>
+            </div>
 
             {!rev ? (
               <div style={{display:'flex',gap:10,justifyContent:'center',maxWidth:340,margin:'8px auto 0'}}>
@@ -678,7 +686,7 @@ export default function App() {
             ) : quickTick ? (
               <QuickTick/>
             ) : (
-              <WechselContextFeedback q={q} inp={inp} ok={ok}/>
+              <WechselContextFeedback q={q} inp={inp} ok={ok} onRule={()=>setRule(true)}/>
             )}
           </div>
         )}
@@ -772,7 +780,8 @@ export default function App() {
         {scr !== 'practice' && <FeatureRequestFooter />}
       </div>
 
-      {chart && <ChartModal section={chartSection} q={q} onClose={()=>setChart(false)}/>}
+      {chart && <ChartModal section={chartSection} q={q} onClose={()=>setChart(false)} onRule={()=>{ setChart(false); setRule(true); }}/>}
+      {rule && <WechselRuleModal onClose={()=>setRule(false)}/>}
 
       <style>{`
         @keyframes shake{0%,100%{transform:translateX(0)}25%{transform:translateX(-6px)}75%{transform:translateX(6px)}}
@@ -1063,17 +1072,135 @@ function PrepFeedback({ q, inp, ok }) {
   );
 }
 
-function WechselContextFeedback({ q, inp, ok }) {
+function WechselContextFeedback({ q, inp, ok, onRule }) {
   return (
     <div style={{...S.feedback, animation:'fadeUp 0.2s ease-out', maxWidth:420}}>
-      <div style={{fontSize:15, fontWeight:600, marginBottom:6, color:ok?GREEN:'#1a1a1a'}}>
+      <div style={{fontSize:15, fontWeight:600, marginBottom:8, color:ok?GREEN:'#1a1a1a'}}>
         {ok ? <>✓ <span style={{color:CASE_C[q.ans]}}>{CL[q.ans]}</span></> : <>✗ <span style={{textDecoration:'line-through',color:'#999',fontWeight:400}}>{CL[inp]||inp}</span> <span style={{color:'#999',fontWeight:400}}>→</span> <span style={{color:CASE_C[q.ans]}}>{CL[q.ans]}</span></>}
       </div>
-      <div style={{fontSize:13, color:'#666', marginBottom:6, lineHeight:1.5}}>
-        „{q.verb}" {q.isMotion ? 'implies motion (wohin?)' : 'implies location (wo?)'} → <span style={{color:CASE_C[q.ans], fontWeight:600}}>{CL[q.ans]}</span>
+      <div style={{fontSize:15, fontWeight:600, color:'#1a1a1a', marginBottom:8, lineHeight:1.45}}>
+        {q.answerSentence}
       </div>
-      <div style={{fontSize:12, color:'#999', marginTop:4, lineHeight:1.6}}>
-        Wechselpräpositionen take <span style={{color:CASE_C.accusative}}>Akkusativ</span> when something moves toward a destination — wohin? (gehen, fahren, legen, stellen) — and <span style={{color:CASE_C.dative}}>Dativ</span> when it stays put — wo? (sein, stehen, sitzen, liegen).
+      {q.contraction && (
+        <div style={{fontSize:12, color:'#999', marginBottom:8}}>
+          {q.prepKey} + {q.article} fuses to <strong style={{color:'#666'}}>{q.contraction}</strong> in real German.
+        </div>
+      )}
+      <div style={{fontSize:13, color:'#666', marginBottom:6, lineHeight:1.5}}>
+        „{q.verb === 'ankommen' ? 'ankommen' : q.verb}" {q.isMotion
+          ? 'puts something somewhere new (wohin?)'
+          : q.verb === 'ankommen'
+          ? 'keeps the Dativ even though you arrive — the an- prefix already carries the arrival'
+          : 'just says where it all happens (wo?)'} → <span style={{color:CASE_C[q.ans], fontWeight:600}}>{CL[q.ans]}</span>
+      </div>
+      <div style={{fontSize:12, color:'#999', lineHeight:1.6}}>
+        Before this happened, was it already {PREPS[q.prepKey].en.split('/')[0]} the {q.nounEn}?
+        {' '}{q.isMotion
+          ? <>No — it got there because of the action. That is <span style={{color:CASE_C.accusative}}>Akkusativ</span>.</>
+          : <>Yes — it was already there and stays. That is <span style={{color:CASE_C.dative}}>Dativ</span>.</>}
+      </div>
+      {onRule && (
+        <button style={{...S.ruleBtn, marginLeft:0, marginTop:10}} onClick={onRule}>ⓘ The whole rule</button>
+      )}
+    </div>
+  );
+}
+
+// The wohin/wo rule, the trap, the verb pairs, and the one real exception.
+function WechselRuleModal({ onClose }) {
+  const PAIRS = [
+    ['legen', 'lay it down', 'liegen', 'be lying'],
+    ['stellen', 'stand it up', 'stehen', 'be standing'],
+    ['setzen', 'sit it down', 'sitzen', 'be sitting'],
+    ['hängen', 'hang it up (hängte)', 'hängen', 'be hanging (hing)'],
+    ['stecken', 'stick it in', 'stecken', 'be stuck in'],
+  ];
+  return (
+    <div style={S.overlay} onClick={onClose}>
+      <div style={S.modal} onClick={e=>e.stopPropagation()}>
+        <div style={{display:'flex',justifyContent:'space-between',alignItems:'center',marginBottom:14}}>
+          <div style={{fontSize:17,fontWeight:700}}>Wohin oder wo?</div>
+          <button onClick={onClose} style={{background:'none',border:'none',fontSize:18,color:'#999',cursor:'pointer'}}>✕</button>
+        </div>
+
+        <div style={S.ruleLead}>
+          Yes — there is a rule, and it is not “is something moving?”. Ask instead:
+          does the phrase say where the thing <strong>ends up</strong>, or where the
+          whole action <strong>happens</strong>?
+        </div>
+
+        <div style={{...S.ruleCard, borderLeft:`3px solid ${CASE_C.accusative}`}}>
+          <div style={{fontWeight:700, color:CASE_C.accusative, fontSize:13, marginBottom:4}}>Akkusativ — wohin?</div>
+          <div style={S.ruleBody}>The action puts something into a new place. It wasn’t there before; it is now.</div>
+          <div style={S.ruleEx}>Er legt das Buch auf <strong>den</strong> Tisch. — the book arrives on the table.</div>
+        </div>
+
+        <div style={{...S.ruleCard, borderLeft:`3px solid ${CASE_C.dative}`}}>
+          <div style={{fontWeight:700, color:CASE_C.dative, fontSize:13, marginBottom:4}}>Dativ — wo?</div>
+          <div style={S.ruleBody}>The phrase just locates the scene. It was already there and stays there.</div>
+          <div style={S.ruleEx}>Das Buch liegt auf <strong>dem</strong> Tisch. — it was already on the table.</div>
+        </div>
+
+        <div style={S.ruleH}>The test that actually works</div>
+        <div style={S.ruleBody}>
+          <strong>“Before this happened, was it already there?”</strong><br/>
+          Already there → <span style={{color:CASE_C.dative,fontWeight:600}}>Dativ</span>.
+          Got there because of the action → <span style={{color:CASE_C.accusative,fontWeight:600}}>Akkusativ</span>.
+        </div>
+
+        <div style={S.ruleH}>The trap: movement is not the test</div>
+        <div style={S.ruleBody}>
+          Plenty of motion verbs take the Dativ, because the phrase is the arena the
+          movement happens in, not a destination:
+        </div>
+        <div style={S.ruleEx}>
+          Er schwimmt in <strong>dem</strong> See (im See) — swimming around <em>in</em> the lake.<br/>
+          Er springt in <strong>den</strong> See — he was out, now he’s in.
+        </div>
+        <div style={S.ruleBody}>Same story with tanzen, spielen, laufen, rennen.</div>
+
+        <div style={S.ruleH}>The verb pairs — learn these cold</div>
+        <div style={{fontSize:11, color:'#999', marginBottom:8, lineHeight:1.5}}>
+          The Akkusativ ones are transitive: they always have a direct object being moved.
+          If something is being put somewhere, the preposition takes Akkusativ too.
+        </div>
+        <table style={S.fullChart}>
+          <thead><tr>
+            <th style={{...S.fcTh, textAlign:'left', color:CASE_C.accusative}}>puts it there · Akk</th>
+            <th style={{...S.fcTh, textAlign:'left', color:CASE_C.dative}}>is already there · Dat</th>
+          </tr></thead>
+          <tbody>
+            {PAIRS.map(([a, aEn, d, dEn]) => (
+              <tr key={a + d}>
+                <td style={{...S.fcTd, textAlign:'left'}}>
+                  <strong>{a}</strong> <span style={{color:'#aaa', fontSize:11}}>{aEn}</span>
+                </td>
+                <td style={{...S.fcTd, textAlign:'left'}}>
+                  <strong>{d}</strong> <span style={{color:'#aaa', fontSize:11}}>{dEn}</span>
+                </td>
+              </tr>
+            ))}
+          </tbody>
+        </table>
+        <div style={{fontSize:11, color:'#999', marginTop:6, lineHeight:1.5}}>
+          hängen and stecken are the mean ones — same present tense either way, so the
+          only clue is whether there is a direct object.
+        </div>
+
+        <div style={S.ruleH}>The exception worth knowing</div>
+        <div style={S.ruleBody}>
+          <strong>ankommen</strong> takes the <span style={{color:CASE_C.dative,fontWeight:600}}>Dativ</span>,
+          even though arriving is the most obvious movement there is:
+        </div>
+        <div style={S.ruleEx}>Wir kommen an <strong>dem</strong> Bahnhof an (am Bahnhof).</div>
+        <div style={S.ruleBody}>
+          The <em>an-</em> prefix already carries the arrival, so the phrase only says where
+          you ended up being. <em>befestigen an</em> + Dativ works the same way.
+        </div>
+
+        <button style={{...S.nextBtn, width:'100%', marginTop:16}} onClick={onClose}>
+          Got it{!IS_TOUCH && <span style={{opacity:0.5,fontSize:12,marginLeft:4}}>esc</span>}
+        </button>
       </div>
     </div>
   );
@@ -1180,7 +1307,7 @@ function PronounFeedback({ q, inp, ok }) {
 // CHART MODAL
 // ═══════════════════════════════════════════
 
-function ChartModal({ section, q, onClose }) {
+function ChartModal({ section, q, onClose, onRule }) {
   const genders = ['masculine','feminine','neuter','plural'];
   const cases = ['nominative','accusative','dative','genitive'];
 
@@ -1251,7 +1378,10 @@ function ChartModal({ section, q, onClose }) {
               <div><strong style={{color:CASE_C.wechsel}}>Wechsel:</strong> {PREPS_BY_CASE.wechsel.join(', ')}</div>
             </div>
             <div style={{fontSize:12, color:'#999', marginTop:14, lineHeight:1.6}}>
-              <strong>Wechsel</strong>: accusative for motion (wohin?), dative for location (wo?).
+              <strong>Wechsel</strong>: Akkusativ when the phrase is where something ends up
+              (wohin?), Dativ when it is where the action happens (wo?) — movement alone does
+              not decide it.
+              {onRule && <button style={{...S.ruleBtn, marginLeft:0, marginTop:8, display:'block'}} onClick={onRule}>ⓘ Wohin oder wo?</button>}
             </div>
           </div>
         )}
@@ -1431,6 +1561,12 @@ const S = {
   choiceBtn: { flex:'1 1 auto', minWidth:90, padding:'14px 16px', borderRadius:10, border:'1.5px solid', background:'#fff', color:'#1a1a1a', cursor:'pointer', fontSize:14, fontFamily:'inherit', textAlign:'center', transition:'all 0.12s',
     ...(IS_TOUCH ? {minHeight:66, display:'flex', flexDirection:'column', alignItems:'center', justifyContent:'center'} : {}) },
 
+  ruleBtn: { marginLeft:8, background:'none', border:'1px solid #ddd', borderRadius:999, padding:'3px 10px', color:'#888', fontSize:11, fontWeight:600, cursor:'pointer', fontFamily:'inherit' },
+  ruleLead: { fontSize:13, color:'#444', lineHeight:1.6, marginBottom:14 },
+  ruleCard: { background:'#fafafa', borderRadius:8, padding:'12px 14px', marginBottom:10 },
+  ruleH: { fontSize:12, fontWeight:800, textTransform:'uppercase', letterSpacing:0.6, color:'#bbb', margin:'18px 0 6px' },
+  ruleBody: { fontSize:13, color:'#555', lineHeight:1.6 },
+  ruleEx: { fontSize:13, color:'#1a1a1a', lineHeight:1.7, background:'#f5f5f5', borderRadius:6, padding:'8px 10px', margin:'8px 0' },
   feedback: { textAlign:'left', background:'#fff', borderRadius:10, padding:'14px 18px', margin:'0 auto', maxWidth:400, border:'1px solid #eee' },
 
   stripTbl: { width:'100%', borderCollapse:'collapse', marginTop:10 },

src/engine.js

@@ -134,6 +134,33 @@ export const NOUNS = [
   {w:'Wetter',g:'neuter',en:'weather',gen:'Wetters',noIndef:true},
   {w:'Dach',g:'neuter',en:'roof',gen:'Daches'},
   {w:'Konzert',g:'neuter',en:'concert',gen:'Konzerts'},
+  {w:'See',g:'masculine',en:'lake',gen:'Sees'},
+  {w:'Fluss',g:'masculine',en:'river',gen:'Flusses'},
+  {w:'Berg',g:'masculine',en:'mountain',gen:'Berges'},
+  {w:'Wald',g:'masculine',en:'forest',gen:'Waldes'},
+  {w:'Bahnhof',g:'masculine',en:'station',gen:'Bahnhofs'},
+  {w:'Flughafen',g:'masculine',en:'airport',gen:'Flughafens'},
+  {w:'Schrank',g:'masculine',en:'cupboard',gen:'Schranks'},
+  {w:'Boden',g:'masculine',en:'floor',gen:'Bodens'},
+  {w:'Supermarkt',g:'masculine',en:'supermarket',gen:'Supermarkts'},
+  {w:'Keller',g:'masculine',en:'cellar',gen:'Kellers'},
+  {w:'Wand',g:'feminine',en:'wall'},
+  {w:'Küche',g:'feminine',en:'kitchen'},
+  {w:'Wohnung',g:'feminine',en:'flat'},
+  {w:'Tafel',g:'feminine',en:'blackboard'},
+  {w:'Wiese',g:'feminine',en:'meadow'},
+  {w:'Tasche',g:'feminine',en:'bag'},
+  {w:'Kirche',g:'feminine',en:'church'},
+  {w:'Bibliothek',g:'feminine',en:'library'},
+  {w:'Bank',g:'feminine',en:'bench'},
+  {w:'Bett',g:'neuter',en:'bed',gen:'Bettes'},
+  {w:'Zimmer',g:'neuter',en:'room',gen:'Zimmers'},
+  {w:'Regal',g:'neuter',en:'shelf',gen:'Regals'},
+  {w:'Sofa',g:'neuter',en:'sofa',gen:'Sofas'},
+  {w:'Meer',g:'neuter',en:'sea',gen:'Meeres'},
+  {w:'Dorf',g:'neuter',en:'village',gen:'Dorfes'},
+  {w:'Bild',g:'neuter',en:'picture',gen:'Bildes'},
+  {w:'Wasser',g:'neuter',en:'water',gen:'Wassers',noIndef:true},
 ];
 const NOUN_BY_NAME = Object.fromEntries(NOUNS.map(n => [n.w, n]));
 export const nounForm = (noun, cas) =>
@@ -146,6 +173,7 @@ const PLACES_IN = ['Park','Garten','Schule','Stadt','Haus','Hotel','Museum','Bü
 const POSSESSABLE = new Set([
   ...PERSONS, 'Hund','Katze','Haus','Garten','Auto','Buch','Tisch','Stuhl',
   'Büro','Schule','Stadt','Fenster','Tür','Musik',
+  'Wohnung','Zimmer','Küche','Tasche','Bett','Sofa','Schrank','Regal','Wand','Bild',
 ]);
 
 export const PRONOUNS = [
@@ -204,6 +232,24 @@ export const VERBS = {
   hängen:  {de:{Er:'hängt',Sie:'hängt',Wir:'hängen',Ich:'hänge'},  en:{Er:'hangs',Sie:'hangs',Wir:'hang',Ich:'hang'}},
   legen:   {de:{Er:'legt',Sie:'legt',Wir:'legen',Ich:'lege'},      en:{Er:'puts',Sie:'puts',Wir:'put',Ich:'put'}},
   stellen: {de:{Er:'stellt',Sie:'stellt',Wir:'stellen',Ich:'stelle'}, en:{Er:'puts',Sie:'puts',Wir:'put',Ich:'put'}},
+  rennen:  {de:{Er:'rennt',Sie:'rennt',Wir:'rennen',Ich:'renne'},      en:{Er:'runs',Sie:'runs',Wir:'run',Ich:'run'}},
+  klettern:{de:{Er:'klettert',Sie:'klettert',Wir:'klettern',Ich:'klettere'}, en:{Er:'climbs',Sie:'climbs',Wir:'climb',Ich:'climb'}},
+  fliegen: {de:{Er:'fliegt',Sie:'fliegt',Wir:'fliegen',Ich:'fliege'},  en:{Er:'flies',Sie:'flies',Wir:'fly',Ich:'fly'}},
+  schwimmen:{de:{Er:'schwimmt',Sie:'schwimmt',Wir:'schwimmen',Ich:'schwimme'}, en:{Er:'swims',Sie:'swims',Wir:'swim',Ich:'swim'}},
+  steigen: {de:{Er:'steigt',Sie:'steigt',Wir:'steigen',Ich:'steige'},  en:{Er:'climbs',Sie:'climbs',Wir:'climb',Ich:'climb'}},
+  werfen:  {de:{Er:'wirft',Sie:'wirft',Wir:'werfen',Ich:'werfe'},      en:{Er:'throws',Sie:'throws',Wir:'throw',Ich:'throw'}},
+  stecken: {de:{Er:'steckt',Sie:'steckt',Wir:'stecken',Ich:'stecke'},  en:{Er:'puts',Sie:'puts',Wir:'put',Ich:'put'}},
+  setzen:  {de:{Er:'setzt',Sie:'setzt',Wir:'setzen',Ich:'setze'},      en:{Er:'sits',Sie:'sits',Wir:'sit',Ich:'sit'}},
+  kleben:  {de:{Er:'klebt',Sie:'klebt',Wir:'kleben',Ich:'klebe'},      en:{Er:'sticks',Sie:'sticks',Wir:'stick',Ich:'stick'}},
+  schreiben:{de:{Er:'schreibt',Sie:'schreibt',Wir:'schreiben',Ich:'schreibe'}, en:{Er:'writes',Sie:'writes',Wir:'write',Ich:'write'}},
+  fallen:  {de:{Er:'fällt',Sie:'fällt',Wir:'fallen',Ich:'falle'},      en:{Er:'falls',Sie:'falls',Wir:'fall',Ich:'fall'}},
+  ziehen:  {de:{Er:'zieht',Sie:'zieht',Wir:'ziehen',Ich:'ziehe'},      en:{Er:'moves',Sie:'moves',Wir:'move',Ich:'move'}},
+  tragen:  {de:{Er:'trägt',Sie:'trägt',Wir:'tragen',Ich:'trage'},      en:{Er:'carries',Sie:'carries',Wir:'carry',Ich:'carry'}},
+  schieben:{de:{Er:'schiebt',Sie:'schiebt',Wir:'schieben',Ich:'schiebe'}, en:{Er:'pushes',Sie:'pushes',Wir:'push',Ich:'push'}},
+  packen:  {de:{Er:'packt',Sie:'packt',Wir:'packen',Ich:'packe'},      en:{Er:'packs',Sie:'packs',Wir:'pack',Ich:'pack'}},
+  tanzen:  {de:{Er:'tanzt',Sie:'tanzt',Wir:'tanzen',Ich:'tanze'},      en:{Er:'is dancing',Sie:'is dancing',Wir:'are dancing',Ich:'am dancing'}},
+  leben:   {de:{Er:'lebt',Sie:'lebt',Wir:'leben',Ich:'lebe'},          en:{Er:'lives',Sie:'lives',Wir:'live',Ich:'live'}},
+  ankommen:{de:{Er:'kommt',Sie:'kommt',Wir:'kommen',Ich:'komme'},      en:{Er:'arrives',Sie:'arrives',Wir:'arrive',Ich:'arrive'}},
 };
 export const SUBJ_EN = {Er:'He',Sie:'She',Wir:'We',Ich:'I'};
 
@@ -307,6 +353,8 @@ export const WECHSEL_SET = new Set(PREPS_BY_CASE.wechsel);
 //   poss      — false: skip this frame for possessive questions
 // Wechsel preps use {motion:[...], location:[...]} instead of a flat list.
 // ═══════════════════════════════════════════
+const ZW_POST = {accusative:' und das Fenster', dative:' und dem Fenster', en:' and the window'};
+
 export const PREP_FRAMES = {
   durch: [{verbs:['gehen','laufen','fahren'], nouns:['Park','Stadt','Straße','Garten','Tür']}],
   für:   [{verbs:['kaufen'], mid:{de:'das Geschenk',en:'the gift'}, nouns:[...PERSONS,'Hund','Katze']}],
@@ -338,69 +386,158 @@ export const PREP_FRAMES = {
   außerhalb: [{verbs:['wohnen','arbeiten'], nouns:['Stadt']}],
   innerhalb: [{verbs:['wohnen','arbeiten'], nouns:['Stadt']}],
 
+  // ── Wechselpräpositionen ────────────────────────────────────
+  // Split by what DECIDES the case, not by "is there movement?".
+  //   motion   → the phrase is the goal: the thing wasn't there, now it is.
+  //   location → the phrase is the arena: the thing was already there.
+  // Motion verbs deliberately appear on the DATIVE side too (schwimmen im See,
+  // tanzen in der Küche, spielen auf der Straße) — that is the whole lesson.
+  // Every frame is one-way: no verb+noun pairing may license both readings,
+  // because level 18 blanks the article and the learner has only the verb.
   an: {
-    motion:  [{verbs:['gehen','laufen'], nouns:['Tür','Fenster']}],
-    location:[{verbs:['stehen','warten','sitzen'], nouns:['Tür','Fenster']}],
+    motion: [
+      {verbs:['gehen','laufen'], nouns:['Tür','Fenster','Tafel']},
+      {verbs:['hängen'], mid:{de:'das Bild',en:'the picture'}, nouns:['Wand','Tür']},
+      {verbs:['schreiben'], mid:{de:'das Wort',en:'the word'}, nouns:['Tafel','Wand']},
+      {verbs:['kleben'], mid:{de:'den Zettel',en:'the note'}, nouns:['Tür','Fenster','Schrank']},
+      {verbs:['stellen'], mid:{de:'den Stuhl',en:'the chair'}, nouns:['Fenster','Wand','Tür']},
+      {verbs:['fahren'], nouns:['See','Fluss','Meer']},
+    ],
+    location: [
+      {verbs:['stehen','warten','sitzen'], nouns:['Tür','Fenster','Tafel']},
+      {verbs:['hängen'], fixedSubj:{de:'Das Bild',en:'The picture'}, nouns:['Wand','Tür'], enVerb:{Er:'is hanging',Sie:'is hanging',Wir:'are hanging',Ich:'am hanging'}},
+      {verbs:['wohnen','leben'], nouns:['See','Fluss']},
+      {verbs:['arbeiten'], nouns:['Tisch','Fenster']},
+      {verbs:['spielen','schlafen'], nouns:['See','Fluss','Meer']},
+      // ankommen takes DATIVE although you obviously arrived — the an- prefix
+      // already carries the arrival, so the phrase only locates it.
+      {verbs:['ankommen'], nouns:['Bahnhof','Flughafen'], post:{dative:' an', en:''}},
+    ],
   },
   auf: {
     motion: [
-      {verbs:['legen'], mid:{de:'das Buch',en:'the book'}, nouns:['Tisch','Stuhl']},
-      {verbs:['gehen'], nouns:['Straße']},
-      {verbs:['springen'], fixedSubj:{de:'Die Katze',en:'The cat'}, nouns:['Tisch','Dach']},
+      {verbs:['legen'], mid:{de:'das Buch',en:'the book'}, nouns:['Tisch','Stuhl','Regal','Bett','Sofa','Boden']},
+      {verbs:['stellen'], mid:{de:'die Lampe',en:'the lamp'}, nouns:['Tisch','Regal','Boden','Schrank']},
+      {verbs:['setzen'], mid:{de:'das Kind',en:'the child'}, nouns:['Stuhl','Sofa','Bank','Bett']},
+      {verbs:['werfen'], mid:{de:'den Ball',en:'the ball'}, nouns:['Dach','Boden','Bett','Sofa']},
+      {verbs:['klettern','steigen'], nouns:['Berg','Baum','Dach','Schrank']},
+      {verbs:['springen'], fixedSubj:{de:'Die Katze',en:'The cat'}, nouns:['Tisch','Dach','Sofa','Schrank']},
+      {verbs:['gehen','laufen'], nouns:['Straße','Wiese','Brücke']},
     ],
     location: [
-      {verbs:['sitzen'], nouns:['Stuhl']},
-      {verbs:['liegen'], fixedSubj:{de:'Das Buch',en:'The book'}, nouns:['Tisch']},
-      {verbs:['stehen','warten'], nouns:['Straße']},
+      {verbs:['sitzen'], nouns:['Stuhl','Sofa','Bank','Boden','Bett']},
+      {verbs:['liegen'], fixedSubj:{de:'Das Buch',en:'The book'}, nouns:['Tisch','Bett','Regal','Boden','Sofa']},
+      {verbs:['stehen','warten'], nouns:['Straße','Dach','Berg','Brücke','Wiese']},
+      {verbs:['spielen','tanzen'], nouns:['Straße','Wiese','Boden','Brücke']},
+      {verbs:['schlafen'], nouns:['Sofa','Boden','Bett']},
+      {verbs:['arbeiten'], nouns:['Dach','Wiese','Straße']},
     ],
   },
   hinter: {
-    motion:  [{verbs:['gehen','laufen'], nouns:['Haus','Baum','Garten','Schule']}],
-    location:[{verbs:['stehen','warten'], nouns:['Haus','Baum','Tür','Schule']}],
+    motion: [
+      {verbs:['gehen','laufen','fahren'], nouns:['Haus','Baum','Garten','Schule','Berg','Kirche']},
+      {verbs:['stellen'], mid:{de:'den Stuhl',en:'the chair'}, nouns:['Tisch','Tür','Schrank','Sofa']},
+      {verbs:['werfen'], mid:{de:'den Ball',en:'the ball'}, nouns:['Haus','Baum','Schrank','Sofa']},
+      {verbs:['legen'], mid:{de:'die Tasche',en:'the bag'}, nouns:['Tür','Schrank','Sofa']},
+    ],
+    location: [
+      {verbs:['stehen','warten'], nouns:['Haus','Baum','Tür','Schule','Kirche','Bahnhof']},
+      {verbs:['liegen'], fixedSubj:{de:'Der Ball',en:'The ball'}, nouns:['Schrank','Sofa','Baum','Haus']},
+      {verbs:['spielen'], nouns:['Haus','Schule','Garten','Kirche']},
+      {verbs:['sitzen'], nouns:['Tisch','Baum','Schrank']},
+      {verbs:['wohnen','leben'], nouns:['Bahnhof','Kirche','Supermarkt','Wald']},
+    ],
   },
   in: {
-    motion: [{verbs:['gehen','fahren','kommen','laufen'], nouns:PLACES_IN}],
+    motion: [
+      {verbs:['gehen','fahren','kommen','laufen','rennen'], nouns:[...PLACES_IN,'Kirche','Bibliothek','Supermarkt','Wald','Dorf','Küche','Zimmer','Keller']},
+      {verbs:['springen','fallen'], nouns:['See','Fluss','Wasser','Meer']},
+      {verbs:['legen'], mid:{de:'das Buch',en:'the book'}, nouns:['Tasche','Schrank','Regal']},
+      {verbs:['stecken'], mid:{de:'den Schlüssel',en:'the key'}, nouns:['Tasche','Tür','Schrank']},
+      {verbs:['packen'], mid:{de:'das Buch',en:'the book'}, nouns:['Tasche','Schrank']},
+      {verbs:['tragen'], mid:{de:'die Tasche',en:'the bag'}, nouns:['Küche','Zimmer','Keller','Haus']},
+      {verbs:['ziehen'], nouns:['Wohnung','Stadt','Dorf','Haus']},
+    ],
     location: [
-      {verbs:['wohnen'], nouns:['Stadt','Haus','Hotel']},
-      {verbs:['sein','warten','sitzen','arbeiten'], nouns:PLACES_IN},
+      {verbs:['wohnen','leben'], nouns:['Stadt','Haus','Hotel','Dorf','Wohnung','Wald']},
+      {verbs:['sein','warten','sitzen','arbeiten'], nouns:[...PLACES_IN,'Kirche','Bibliothek','Supermarkt','Küche','Zimmer','Keller']},
+      {verbs:['schwimmen'], nouns:['See','Fluss','Meer','Wasser']},
+      {verbs:['tanzen','spielen'], nouns:['Küche','Zimmer','Garten','Wohnung','Wald']},
+      {verbs:['schlafen'], nouns:['Bett','Zimmer','Hotel','Keller']},
+      {verbs:['liegen'], fixedSubj:{de:'Das Buch',en:'The book'}, nouns:['Tasche','Schrank','Regal']},
+      {verbs:['stecken'], fixedSubj:{de:'Der Schlüssel',en:'The key'}, nouns:['Tür','Tasche','Schrank'], enVerb:{Er:'is stuck',Sie:'is stuck',Wir:'are stuck',Ich:'am stuck'}},
+      {verbs:['ankommen'], nouns:['Stadt','Hotel','Dorf'], post:{dative:' an', en:''}},
     ],
   },
   neben: {
     motion: [
-      {verbs:['stellen'], mid:{de:'die Lampe',en:'the lamp'}, nouns:['Tisch','Stuhl','Tür']},
-      {verbs:['legen'], mid:{de:'das Buch',en:'the book'}, nouns:['Stuhl','Tisch']},
+      {verbs:['stellen'], mid:{de:'die Lampe',en:'the lamp'}, nouns:['Tisch','Stuhl','Tür','Bett','Sofa','Schrank']},
+      {verbs:['legen'], mid:{de:'das Buch',en:'the book'}, nouns:['Stuhl','Tisch','Bett','Sofa']},
+      {verbs:['setzen'], mid:{de:'das Kind',en:'the child'}, nouns:['Fenster','Tür','Schrank']},
+      {verbs:['schieben'], mid:{de:'den Stuhl',en:'the chair'}, nouns:['Tisch','Bett','Regal']},
     ],
     location: [
-      {verbs:['stehen','sitzen'], nouns:['Tisch','Tür','Fenster']},
-      {verbs:['wohnen'], nouns:['Schule','Park','Museum']},
+      {verbs:['stehen','sitzen'], nouns:['Tisch','Tür','Fenster','Bett','Schrank','Sofa']},
+      {verbs:['wohnen','leben'], nouns:['Schule','Park','Museum','Kirche','Bahnhof','Supermarkt','Bibliothek']},
+      {verbs:['liegen'], fixedSubj:{de:'Das Buch',en:'The book'}, nouns:['Bett','Sofa','Tisch','Regal']},
+      {verbs:['warten'], nouns:['Tür','Bahnhof','Kirche']},
+      {verbs:['arbeiten'], nouns:['Fenster','Tür']},
     ],
   },
   über: {
     motion: [
-      {verbs:['gehen','laufen'], nouns:['Straße','Brücke']},
-      {verbs:['hängen'], mid:{de:'die Lampe',en:'the lamp'}, nouns:['Tisch']},
+      {verbs:['gehen','laufen','fahren','rennen'], nouns:['Straße','Brücke','Wiese']},
+      {verbs:['hängen'], mid:{de:'die Lampe',en:'the lamp'}, nouns:['Tisch','Bett','Sofa']},
+      {verbs:['fliegen'], nouns:['Stadt','Berg','Meer','Wald','See']},
+      {verbs:['werfen'], mid:{de:'den Ball',en:'the ball'}, nouns:['Wand','Brücke','Schrank']},
+    ],
+    location: [
+      {verbs:['hängen'], fixedSubj:{de:'Die Lampe',en:'The lamp'}, nouns:['Tisch','Bett','Sofa'], enVerb:{Er:'is hanging',Sie:'is hanging',Wir:'are hanging',Ich:'am hanging'}},
+      {verbs:['wohnen','leben'], nouns:['Restaurant','Supermarkt','Büro','Bibliothek']},
+      {verbs:['arbeiten'], nouns:['Restaurant','Büro','Supermarkt']},
     ],
-    location:[{verbs:['hängen'], fixedSubj:{de:'Die Lampe',en:'The lamp'}, nouns:['Tisch']}],
   },
   unter: {
     motion: [
-      {verbs:['legen'], mid:{de:'das Buch',en:'the book'}, nouns:['Tisch','Stuhl']},
-      {verbs:['laufen'], fixedSubj:{de:'Die Katze',en:'The cat'}, nouns:['Tisch','Baum']},
+      {verbs:['legen'], mid:{de:'das Buch',en:'the book'}, nouns:['Tisch','Stuhl','Bett','Sofa','Regal']},
+      {verbs:['stellen'], mid:{de:'die Tasche',en:'the bag'}, nouns:['Tisch','Bett','Stuhl','Regal']},
+      {verbs:['werfen'], mid:{de:'den Ball',en:'the ball'}, nouns:['Sofa','Bett','Tisch','Schrank']},
+      {verbs:['schieben'], mid:{de:'die Tasche',en:'the bag'}, nouns:['Bett','Sofa','Tisch']},
+      {verbs:['laufen','rennen'], fixedSubj:{de:'Die Katze',en:'The cat'}, nouns:['Tisch','Baum','Bett','Sofa']},
     ],
     location: [
-      {verbs:['sitzen','stehen','schlafen'], nouns:['Baum','Brücke']},
-      {verbs:['schlafen'], fixedSubj:{de:'Die Katze',en:'The cat'}, nouns:['Tisch','Stuhl']},
+      {verbs:['sitzen','stehen','schlafen'], nouns:['Baum','Brücke','Dach']},
+      {verbs:['liegen'], fixedSubj:{de:'Das Buch',en:'The book'}, nouns:['Bett','Sofa','Tisch','Stuhl','Regal']},
+      {verbs:['spielen','tanzen'], nouns:['Baum','Brücke','Dach']},
+      {verbs:['warten'], nouns:['Brücke','Baum','Dach']},
+      {verbs:['wohnen'], nouns:['Dach']},
     ],
   },
   vor: {
-    motion:  [{verbs:['fahren','gehen'], nouns:['Haus','Tür','Schule','Hotel']}],
-    location:[{verbs:['stehen','warten'], nouns:['Tür','Haus','Schule','Hotel','Museum','Restaurant']}],
+    motion: [
+      {verbs:['fahren','gehen','laufen'], nouns:['Haus','Tür','Schule','Hotel','Kirche','Bahnhof','Supermarkt']},
+      {verbs:['stellen'], mid:{de:'den Stuhl',en:'the chair'}, nouns:['Fenster','Tür','Schrank','Regal']},
+      {verbs:['legen'], mid:{de:'die Tasche',en:'the bag'}, nouns:['Tür','Bett','Sofa']},
+    ],
+    location: [
+      {verbs:['stehen','warten'], nouns:['Tür','Haus','Schule','Hotel','Museum','Restaurant','Kirche','Bahnhof','Supermarkt','Bibliothek']},
+      {verbs:['sitzen'], nouns:['Fenster','Haus','Tür']},
+      {verbs:['spielen'], nouns:['Haus','Schule','Kirche','Garten']},
+      {verbs:['tanzen'], nouns:['Haus','Kirche','Hotel']},
+      {verbs:['liegen'], fixedSubj:{de:'Die Tasche',en:'The bag'}, nouns:['Tür','Bett','Sofa','Schrank']},
+    ],
   },
   zwischen: {
-    motion: [{verbs:['stellen'], mid:{de:'die Lampe',en:'the lamp'}, nouns:['Tisch','Stuhl'],
-              post:{accusative:' und das Fenster', dative:' und dem Fenster', en:' and the window'}}],
-    location: [{verbs:['stehen','sitzen'], nouns:['Tisch','Stuhl'],
-              post:{accusative:' und das Fenster', dative:' und dem Fenster', en:' and the window'}}],
+    motion: [
+      {verbs:['stellen'], mid:{de:'die Lampe',en:'the lamp'}, nouns:['Tisch','Stuhl','Sofa','Schrank'], post:ZW_POST},
+      {verbs:['legen'], mid:{de:'das Buch',en:'the book'}, nouns:['Tisch','Stuhl','Bett'], post:ZW_POST},
+      {verbs:['setzen'], mid:{de:'das Kind',en:'the child'}, nouns:['Stuhl','Sofa'], post:ZW_POST},
+    ],
+    location: [
+      {verbs:['stehen','sitzen'], nouns:['Tisch','Stuhl','Sofa','Schrank'], post:ZW_POST},
+      {verbs:['liegen'], fixedSubj:{de:'Das Buch',en:'The book'}, nouns:['Tisch','Stuhl','Regal'], post:ZW_POST},
+      {verbs:['warten'], nouns:['Tisch','Schrank'], post:ZW_POST},
+    ],
   },
 };
 
@@ -644,9 +781,18 @@ export function genWechselContextQ(level, state = newState()) {
   const cas = isMotion ? 'accusative' : 'dative';
   const r = realizeFrame(prepKey, cas, isMotion, state);
   const art = DEF[r.noun.g][cas];
-  const sentence = `${r.subjDe} ${r.verbDe}${r.midDe} ${prepKey} ${art} ${nounForm(r.noun, cas)}${r.postDe}.`;
+  const head = `${r.subjDe} ${r.verbDe}${r.midDe}`;
+  const tail = `${nounForm(r.noun, cas)}${r.postDe}.`;
+  // The article is BLANKED. With "unter dem Stuhl" on screen the answer is just
+  // article recognition; blanked, the only evidence is the verb (and whether it
+  // has a direct object) — which is the actual wohin/wo skill.
+  const sentence = `${head} ${prepKey} _____ ${tail}`;
+  // The solved sentence contracts where German really contracts (in dem → im).
+  const fused = CONTRACTABLE[prepKey]?.[art] || null;
+  const answerSentence = fused ? `${head} ${fused} ${tail}` : `${head} ${prepKey} ${art} ${tail}`;
   const translation = `${r.subjEn} ${r.verbEn}${r.midEn} ${r.enPrep} ${enNoun(r, r.enDet)}${r.postEn}.`;
-  return {type:'wechselContext', prepKey, ans:cas, isMotion, verb:r.verbKey, sentence, translation, nounEn:r.noun.en};
+  return {type:'wechselContext', prepKey, ans:cas, isMotion, verb:r.verbKey, noun:r.noun,
+    article:art, contraction:fused, sentence, answerSentence, translation, nounEn:r.noun.en};
 }
 
 const POSS_ACC_TEMPLATES = [

src/engine.test.js

@@ -3,7 +3,7 @@ import {
   DEF, INDEF, PREPS, PREPS_BY_CASE, COMMON_PREPS, RARE_PREPS, ALL_PREPS,
   NOUNS, nounForm, LEVELS, CONTRACTIONS, CONTRACTABLE, CHUNK_PREPS,
   POSS_ENDINGS, combinePoss, PRONOUNS, VERBS, PREP_FRAMES,
-  createSession, gen, genContractionContextQ, newState, buildChunkChoices,
+  createSession, gen, genContractionContextQ, newState, buildChunkChoices, PREPS_BY_CASE as _PBC,
   bumpStreak, currentStreak, weekDots,
 } from './engine.js';
 
@@ -189,6 +189,75 @@ describe('wechsel in context (level 18)', () => {
       expect(q.sentence.endsWith('.')).toBe(true);
     }
   });
+
+  it('blanks the article so the verb is the only evidence', () => {
+    for (const q of runSession(18, 300)) {
+      expect(q.sentence).toContain('_____');
+      expect(q.article).toBe(DEF[q.noun.g][q.ans]);
+      // the blanked sentence must never leak the answer article next to the prep
+      expect(q.sentence).not.toContain(` ${q.prepKey} ${q.article} `);
+    }
+  });
+
+  it('the solved sentence contracts exactly where German contracts', () => {
+    for (const q of runSession(18, 400)) {
+      const fused = CONTRACTABLE[q.prepKey]?.[q.article] || null;
+      expect(q.contraction).toBe(fused);
+      expect(q.answerSentence).not.toContain('_____');
+      if (fused) expect(q.answerSentence).toContain(` ${fused} `);
+      else expect(q.answerSentence).toContain(` ${q.prepKey} ${q.article} `);
+    }
+  });
+
+  it('drills far more than the handful of cats and lamps it used to', () => {
+    const qs = runSession(18, 400);
+    expect(new Set(qs.map(q => q.verb)).size).toBeGreaterThanOrEqual(20);
+    expect(new Set(qs.map(q => q.noun.w)).size).toBeGreaterThanOrEqual(25);
+    // no single sentence should dominate the round
+    const counts = {};
+    for (const q of qs) counts[q.answerSentence] = (counts[q.answerSentence] || 0) + 1;
+    expect(Math.max(...Object.values(counts))).toBeLessThanOrEqual(12);
+  });
+
+  it('teaches that movement alone is not Akkusativ', () => {
+    const qs = runSession(18, 600);
+    // motion verbs used with a dative "arena" reading
+    const arena = qs.filter(q => ['schwimmen','tanzen','spielen','fliegen'].includes(q.verb) && q.ans === 'dative');
+    expect(arena.length).toBeGreaterThan(0);
+    // ankommen is the lexical exception: always dative
+    const ank = qs.filter(q => q.verb === 'ankommen');
+    expect(ank.length).toBeGreaterThan(0);
+    for (const q of ank) expect(q.ans).toBe('dative');
+  });
+
+  it('the put/be verb pairs land on the case their transitivity demands', () => {
+    // Unambiguously transitive "put it there" verbs vs unambiguously stative ones.
+    // hängen and stecken are deliberately excluded — see the next test.
+    const PUT = ['legen','stellen','setzen','werfen','schieben','packen','tragen','kleben','schreiben'];
+    const BE  = ['liegen','stehen','sitzen','wohnen','leben','schlafen','warten','arbeiten','sein'];
+    for (const q of runSession(18, 600)) {
+      if (PUT.includes(q.verb)) expect(q.ans, `${q.answerSentence}`).toBe('accusative');
+      if (BE.includes(q.verb)) expect(q.ans, `${q.answerSentence}`).toBe('dative');
+    }
+  });
+
+  it('hängen and stecken appear in BOTH cases — only the direct object decides', () => {
+    const qs = runSession(18, 800);
+    for (const v of ['hängen', 'stecken']) {
+      const mine = qs.filter(q => q.verb === v);
+      expect(mine.some(q => q.ans === 'accusative'), `${v} never showed transitive`).toBe(true);
+      expect(mine.some(q => q.ans === 'dative'), `${v} never showed intransitive`).toBe(true);
+      // transitive = something is being moved, so the sentence carries an object
+      // before the preposition; intransitive sentences start with that thing.
+      for (const q of mine) {
+        const beforePrep = q.answerSentence.split(` ${q.prepKey} `)[0];
+        const fusedHead = q.contraction ? q.answerSentence.split(` ${q.contraction} `)[0] : beforePrep;
+        const head = q.contraction ? fusedHead : beforePrep;
+        const hasObject = /\b(das|den|die|dem)\b/.test(head);
+        expect(hasObject, `${q.answerSentence} (${q.ans})`).toBe(q.ans === 'accusative');
+      }
+    }
+  });
 });
 
 // ── Chunk / contraction questions ───────────────────────────