Once you know that a ㅡ stem drops its ㅡ before -아/어, a second question appears the moment the stem has more than one syllable: after the ㅡ is gone, do you attach 아 or 어? 바쁘다 ("to be busy") becomes 바빠요, but 예쁘다 ("to be pretty") becomes 예뻐요 — same ㅡ-drop, opposite ending. The two verbs differ by a single vowel at the front, and that vowel is exactly what settles the choice. This page gives you the one lookback rule that makes multi-syllable ㅡ stems as predictable as the one-syllable ones.
The rule: look at the syllable before the ㅡ
When a multi-syllable ㅡ stem drops its ㅡ, vowel harmony still applies — but since the ㅡ itself is now gone, harmony looks past the empty slot to the vowel of the immediately preceding syllable:
- If that vowel is ㅏ or ㅗ (the bright vowels) → use 아.
- For anything else → use 어.
바쁘 → drop ㅡ → look at 바 (ㅏ, bright) → 아 → 바 + 빠 → 바빠
예쁘 → drop ㅡ → look at 예 (ㅖ, not bright) → 어 → 예 + 뻐 → 예뻐
The two stems are structurally identical — [consonant]ㅏ/ㅖ + 쁘 — so the only thing that can flip the output is that first vowel, and it does.
요즘 일이 많아서 너무 바빠요.
yojeum iri manaseo neomu bappayo
I'm so busy these days because I have a lot of work. (바쁘다 → 바빠요)
그 원피스 정말 예뻐요.
geu wonpiseu jeongmal yeppeoyo
That dress is really pretty. (예쁘다 → 예뻐요)
Bright-lookback stems → 아
If the syllable before the ㅡ carries ㅏ or ㅗ, the ending is 아:
| Verb | Meaning | Lookback vowel | -아/어 form |
|---|---|---|---|
| 바쁘다 | be busy | 바 (ㅏ) | 바빠요 |
| 아프다 | hurt / be sick | 아 (ㅏ) | 아파요 |
| 나쁘다 | be bad | 나 (ㅏ) | 나빠요 |
| 고프다 | be hungry (배고프다) | 고 (ㅗ) | 고파요 |
| 모으다 | gather / collect | 모 (ㅗ) | 모아요 |
| 잠그다 | lock / turn off (tap) | 잠 (ㅏ) | 잠가요 |
어제부터 목이 아파요.
eojebuteo mogi apayo
My throat has been sore since yesterday. (아프다 → 아파요)
아침을 안 먹어서 배가 고파요.
achimeul an meogeoseo baega gopayo
I'm hungry because I skipped breakfast. (배고프다 → 고파요)
저금통에 동전을 모아요.
jeogeumtong-e dongjeoneul moayo
I collect coins in a piggy bank. (모으다 → 모아요)
Note 모으다: the ㅡ syllable (으) has no consonant, so dropping the ㅡ leaves nothing but the bright lookback vowel — 모 + 아 → 모아. The rule doesn't care whether the ㅡ syllable had a consonant; it only cares about the vowel one step back.
Everything-else stems → 어
If the lookback vowel is anything other than ㅏ or ㅗ — ㅕ, ㅣ, ㅡ, ㅜ, ㅔ, and the rest — the ending is 어:
| Verb | Meaning | Lookback vowel | -아/어 form |
|---|---|---|---|
| 예쁘다 | be pretty | 예 (ㅖ) | 예뻐요 |
| 기쁘다 | be glad | 기 (ㅣ) | 기뻐요 |
| 슬프다 | be sad | 슬 (ㅡ) | 슬퍼요 |
합격 소식을 듣고 정말 기뻐요.
hapgyeok sosigeul deutgo jeongmal gippeoyo
I'm so glad to hear I passed. (기쁘다 → 기뻐요)
영화가 너무 슬퍼서 펑펑 울었어요.
yeonghwaga neomu seulpeoseo peongpeong ureosseoyo
The movie was so sad I bawled my eyes out. (슬프다 → 슬퍼서)
날씨가 나빠서 등산을 취소했어요.
nalssiga nappaseo deungsaneul chwisohaesseoyo
The weather was bad so we canceled the hike. (나쁘다 → 나빠서)
며칠 동안 너무 아팠어요.
myeochil dong-an neomu apasseoyo
I was really sick for a few days. (아프다, past → 아팠어요)
The clean contrast: 바빠요 vs 예뻐요
Set the two flagship verbs side by side. Everything is identical except the front vowel, and the whole output tracks it:
| 바쁘다 | 예쁘다 | |
|---|---|---|
| lookback vowel | 바 = ㅏ (bright) | 예 = ㅖ (not bright) |
| ending picked | 아 | 어 |
| present | 바빠요 | 예뻐요 |
| past | 바빴어요 | 예뻤어요 |
| -아/어서 | 바빠서 | 예뻐서 |
Contrast with one-syllable stems
The lookback rule is what separates multi-syllable stems from the one-syllable ㅡ stems. A stem like 쓰다 or 크다 has no preceding syllable to look back to, so harmony has nothing to read and defaults to 어: 써요, 커요. The moment a second syllable exists, that default is overridden by the lookback vowel — which is why 바쁘다 breaks the "ㅡ stems take 어" expectation and comes out 바빠요.
Reframing for English speakers
The counter-intuitive part for English speakers is that a "deleted" element still governs the choice around it — you pick 아 vs 어 based on a vowel that is two syllables from the ending, across an empty slot the ㅡ used to fill. English has nothing like this action-at-a-distance in its morphology. The mental fix is to stop treating the ㅡ as a vowel that "should" pick the ending, and instead treat it as a spacer that pops out and lets the real stem vowel — the one before it — do the harmonizing, exactly as it would in any ordinary -아/어 verb. Once you see 바쁘 as "really a ㅏ-stem wearing a ㅡ costume," 바빠요 stops feeling irregular at all.
Note that 르-stems use this same lookback for their 라 vs 러 (다르다 → 달라, 흐르다 → 흘러), just with the extra ㄹ-doubling layered on top — so the harmony instinct you build here transfers directly.
Common Mistakes
1. Defaulting a bright-lookback stem to 어. The deleted ㅡ does not force 어 — the earlier ㅏ decides.
❌ 요즘 너무 바뻐요.
Wrong — after the ㅡ drops, the earlier ㅏ (bright) picks 아 → 바빠요.
✅ 요즘 너무 바빠요.
yojeum neomu bappayo
I'm so busy these days.
2. Forcing 아 onto a dark-lookback stem. 예쁘다 looks back to ㅖ, which is not bright.
❌ 그 옷 정말 예빠요.
Wrong — the lookback vowel ㅖ is not bright → 예뻐요, not ×예빠요.
✅ 그 옷 정말 예뻐요.
geu ot jeongmal yeppeoyo
That outfit is really pretty.
3. Reading 아프다 as dark. The lookback vowel is ㅏ — bright.
❌ 배가 너무 아퍼요.
Wrong — the earlier ㅏ is bright → 아파요, not ×아퍼요.
✅ 배가 너무 아파요.
baega neomu apayo
My stomach really hurts.
4. Forcing 아 onto 슬프다. The lookback vowel ㅡ is not bright.
❌ 영화가 너무 슬파요.
Wrong — the earlier ㅡ isn't bright → 슬퍼요, not ×슬파요.
✅ 영화가 너무 슬퍼요.
yeonghwaga neomu seulpeoyo
The movie is so sad.
Key Takeaways
- After a multi-syllable ㅡ stem drops its ㅡ, the vowel of the syllable immediately before the ㅡ picks the ending: ㅏ/ㅗ → 아, anything else → 어.
- 바쁘다 → 바빠요 (바 has ㅏ); 예쁘다 → 예뻐요 (예 is not bright). The two differ only in that front vowel.
- The rule is the same harmony you use everywhere, just reading one syllable earlier because the ㅡ is deleted.
- One-syllable ㅡ stems have no lookback vowel and so always default to 어 (커요, 써요) — a separate page.
- 르-stems use the identical lookback for 라 vs 러, plus a ㄹ-doubling twist.
Now practice Korean
Reading grammar gets you part of the way. The exercises are where it sticks — free, no signup needed.
Start learning Korean→Related Topics
- The 으 Drop: 쓰다 → 써요, 크다 → 커요TOPIK 1 — Any stem whose last vowel is ㅡ loses that ㅡ before an -아/어 ending. For a one-syllable ㅡ stem there is no preceding vowel, so it always defaults to 어: 쓰다 → 써요, 크다 → 커요, 끄다 → 꺼요. The most predictable of all the 'irregular' classes.
- Vowel Harmony: Choosing -아 vs -어TOPIK 1 — One rule fixes the shape of every -아/어 ending: if the stem's LAST vowel is ㅏ or ㅗ (bright), use 아; for anything else, use 어. The single memorized exception is 하다 → 해.
- The 르 Irregular: 모르다 → 몰라요TOPIK 1 — The high-frequency 르 irregular — before an 아/어 ending the 으 of 르 drops and an extra ㄹ pushes back onto the previous syllable (모르다 → 몰라요, 빠르다 → 빨라요), with 라/러 chosen by vowel harmony.
- Irregular Predicates at a Glance (Reference Table)TOPIK 2 — One-screen reference for all eight irregular classes — the trigger, the change, a model verb with its 아/어-form and 으-form, and a regular look-alike to guard against over-generalizing each class.