diff --git a/src/engine.js b/src/engine.js
index fbe8ceb..4783d7a 100644
--- a/src/engine.js
+++ b/src/engine.js
@@ -89,6 +89,9 @@ export function wechselExample(prepKey, isMotion) {
 }
 
 // gen: genitive noun form for masculine/neuter (des Mannes, des Hauses)
+// noIndef: never combine with ein-words — mass nouns („eines Wetters" is not
+// German) and nouns whose frames only read naturally with a definite article
+// (kinship terms, temporal Nacht/Woche: „während einer Woche" is off).
 export const NOUNS = [
   {w:'Hund',g:'masculine',en:'dog',gen:'Hundes'},
   {w:'Mann',g:'masculine',en:'man',gen:'Mannes'},
@@ -99,24 +102,24 @@ export const NOUNS = [
   {w:'Baum',g:'masculine',en:'tree',gen:'Baumes'},
   {w:'Garten',g:'masculine',en:'garden',gen:'Gartens'},
   {w:'Stuhl',g:'masculine',en:'chair',gen:'Stuhls'},
-  {w:'Bruder',g:'masculine',en:'brother',gen:'Bruders'},
+  {w:'Bruder',g:'masculine',en:'brother',gen:'Bruders',noIndef:true},
   {w:'Lehrer',g:'masculine',en:'teacher',gen:'Lehrers'},
-  {w:'Vater',g:'masculine',en:'father',gen:'Vaters'},
+  {w:'Vater',g:'masculine',en:'father',gen:'Vaters',noIndef:true},
   {w:'Zug',g:'masculine',en:'train',gen:'Zuges'},
-  {w:'Regen',g:'masculine',en:'rain',gen:'Regens'},
+  {w:'Regen',g:'masculine',en:'rain',gen:'Regens',noIndef:true},
   {w:'Frau',g:'feminine',en:'woman'},
   {w:'Katze',g:'feminine',en:'cat'},
   {w:'Schule',g:'feminine',en:'school'},
   {w:'Stadt',g:'feminine',en:'city'},
   {w:'Straße',g:'feminine',en:'street'},
   {w:'Tür',g:'feminine',en:'door'},
-  {w:'Schwester',g:'feminine',en:'sister'},
-  {w:'Mutter',g:'feminine',en:'mother'},
-  {w:'Tochter',g:'feminine',en:'daughter'},
-  {w:'Musik',g:'feminine',en:'music'},
-  {w:'Nacht',g:'feminine',en:'night'},
-  {w:'Woche',g:'feminine',en:'week'},
-  {w:'Arbeit',g:'feminine',en:'work'},
+  {w:'Schwester',g:'feminine',en:'sister',noIndef:true},
+  {w:'Mutter',g:'feminine',en:'mother',noIndef:true},
+  {w:'Tochter',g:'feminine',en:'daughter',noIndef:true},
+  {w:'Musik',g:'feminine',en:'music',noIndef:true},
+  {w:'Nacht',g:'feminine',en:'night',noIndef:true},
+  {w:'Woche',g:'feminine',en:'week',noIndef:true},
+  {w:'Arbeit',g:'feminine',en:'work',noIndef:true},
   {w:'Brücke',g:'feminine',en:'bridge'},
   {w:'Kind',g:'neuter',en:'child',gen:'Kindes'},
   {w:'Haus',g:'neuter',en:'house',gen:'Hauses'},
@@ -128,7 +131,7 @@ export const NOUNS = [
   {w:'Museum',g:'neuter',en:'museum',gen:'Museums'},
   {w:'Büro',g:'neuter',en:'office',gen:'Büros'},
   {w:'Restaurant',g:'neuter',en:'restaurant',gen:'Restaurants'},
-  {w:'Wetter',g:'neuter',en:'weather',gen:'Wetters'},
+  {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'},
 ];
@@ -480,7 +483,7 @@ function framesFor(prepKey, tw) {
 
 // Resolve a frame into concrete sentence parts. Returns null if the
 // preposition has no usable frame for the given constraints.
-function realizeFrame(prepKey, cas, tw, state, { genderFilter, possOnly } = {}) {
+function realizeFrame(prepKey, cas, tw, state, { genderFilter, possOnly, indefOnly } = {}) {
   let frames = framesFor(prepKey, tw);
   if (possOnly) {
     frames = frames
@@ -488,6 +491,15 @@ function realizeFrame(prepKey, cas, tw, state, { genderFilter, possOnly } = {})
       .map(f => ({...f, nouns: f.nouns.filter(n => POSSESSABLE.has(n))}))
       .filter(f => f.nouns.length);
   }
+  if (indefOnly) {
+    // ein-word questions: drop frames whose English determiner is a baked-in
+    // definite idiom (nach der Schule → "after school", seit → "since that…"),
+    // and drop noIndef nouns — the remaining pairings read naturally with a/an.
+    frames = frames
+      .filter(f => f.enDet === undefined)
+      .map(f => ({...f, nouns: f.nouns.filter(n => !NOUN_BY_NAME[n].noIndef)}))
+      .filter(f => f.nouns.length);
+  }
   if (genderFilter) {
     frames = frames
       .map(f => ({...f, nouns: f.nouns.filter(n => genderFilter.includes(NOUN_BY_NAME[n].g))}))
@@ -550,24 +562,35 @@ function articleChoices(ans, artType) {
 
 export function genArticleQ(level, state = newState()) {
   const lv = LEVELS.find(l=>l.id===level);
-  const prepKey = drawBag(state, 'prep', lv.preps, state.last.prep);
-  state.last.prep = prepKey;
-  const pc = PREPS[prepKey].c;
-  let tw = null, cas;
-  if (pc === 'wechsel') { tw = Math.random() > 0.5; cas = tw ? 'accusative' : 'dative'; }
-  else cas = pc;
+  const indef = lv.artType === 'indefinite';
+  // Draw preps until one yields a usable frame. With ein-words, mass nouns and
+  // definite idioms can rule a preposition out entirely (e.g. trotz, whose
+  // nouns are all uncountable — „trotz eines Wetters" is not German).
+  let prepKey, cas, tw = null, r = null;
+  for (let i = 0; i <= lv.preps.length && !r; i++) {
+    prepKey = drawBag(state, 'prep', lv.preps, state.last.prep);
+    const pc = PREPS[prepKey].c;
+    if (pc === 'wechsel') { tw = Math.random() > 0.5; cas = tw ? 'accusative' : 'dative'; }
+    else { tw = null; cas = pc; }
 
-  // Mixed definite lessons: contraction-capable prep → fused "chunk" question.
-  if (lv.mixed && lv.artType === 'definite' && CHUNK_PREPS.has(prepKey)) {
-    return makeChunkQ(prepKey, cas, tw, state);
+    // Mixed definite lessons: contraction-capable prep → fused "chunk" question.
+    if (lv.mixed && lv.artType === 'definite' && CHUNK_PREPS.has(prepKey)) {
+      state.last.prep = prepKey;
+      return makeChunkQ(prepKey, cas, tw, state);
+    }
+
+    r = realizeFrame(prepKey, cas, tw, state, { indefOnly: indef });
   }
+  if (!r) r = realizeFrame(prepKey, cas, tw, state); // hard fallback; unreachable with current data
+  state.last.prep = prepKey;
 
-  const r = realizeFrame(prepKey, cas, tw, state);
-  const tbl = lv.artType === 'indefinite' ? INDEF : DEF;
+  const tbl = indef ? INDEF : DEF;
   const ans = tbl[r.noun.g][cas];
+  // ein-words translate as a/an, never the frame's default "the".
+  const detEn = indef ? (/^[aeiou]/i.test(r.noun.en) ? 'an' : 'a') : r.enDet;
   const sentence = `${r.subjDe} ${r.verbDe}${r.midDe} ${prepKey} _____ ${nounForm(r.noun, cas)}${r.postDe}.`;
-  const translation = `${r.subjEn} ${r.verbEn}${r.midEn} ${r.enPrep} ${enNoun(r, r.enDet)}${r.postEn}.`;
-  const rule = pc === 'wechsel'
+  const translation = `${r.subjEn} ${r.verbEn}${r.midEn} ${r.enPrep} ${enNoun(r, detEn)}${r.postEn}.`;
+  const rule = PREPS[prepKey].c === 'wechsel'
     ? (tw ? `„${prepKey}" + motion (wohin?) → Akkusativ` : `„${prepKey}" + location (wo?) → Dativ`)
     : `„${prepKey}" → always ${CL[cas]}`;
   return {type:'article', noun:r.noun, cas, prepKey, ans, sentence, translation, rule, tw, artType:lv.artType, choices:articleChoices(ans, lv.artType)};
diff --git a/src/engine.test.js b/src/engine.test.js
index 4872a01..b49d76d 100644
--- a/src/engine.test.js
+++ b/src/engine.test.js
@@ -109,6 +109,32 @@ describe('article questions', () => {
     const seen = new Set(runSession(3, lv.preps.length).map(q => q.prepKey));
     expect([...seen].sort()).toEqual([...lv.preps].sort());
   });
+
+  it('ein-Wörter never pairs an ein-word with a mass/definite-only noun', () => {
+    for (const q of runSession(7, 400)) {
+      expect(q.noun.noIndef, `„ein ${q.noun.w}" should never be generated`).toBeFalsy();
+    }
+  });
+
+  it('ein-Wörter never uses definite-idiom frames (nach, seit) or all-mass preps (trotz)', () => {
+    for (const q of runSession(7, 400)) {
+      expect(['nach', 'seit', 'trotz']).not.toContain(q.prepKey);
+    }
+  });
+
+  it('ein-Wörter translations say a/an, matching the German article', () => {
+    for (const q of runSession(7, 400)) {
+      const usesA = q.translation.includes(` a ${q.noun.en}`) || q.translation.includes(` an ${q.noun.en}`);
+      expect(usesA, `"${q.translation}" should use a/an for „${q.ans} ${q.noun.w}"`).toBe(true);
+      expect(q.translation).not.toContain(` the ${q.noun.en}`);
+    }
+  });
+
+  it('definite levels still translate with "the"', () => {
+    for (const q of runSession(2, 200)) {
+      expect(q.translation).toContain(` the ${q.noun.en}`);
+    }
+  });
 });
 
 // ── Preposition case quiz ───────────────────────────────────