The 으 drop is the class textbooks file under "irregular" but that you should think of as the exact opposite: it is the most predictable conjugation in Korean. The rule is one line — any stem whose final vowel is ㅡ loses that ㅡ before an -아/어 ending — and unlike the ㅂ or ㄷ classes, there are no exceptions to memorize. If you can spot a ㅡ in the last syllable of a stem, you can conjugate the verb on sight. This page covers the one-syllable case (which always defaults to 어); the multi-syllable case, where the choice of 아 vs 어 requires one extra step, gets its own page.
The rule: ㅡ vanishes before -아/어
A stem like 쓰- ends in the vowel ㅡ (쓰 = ㅆ + ㅡ). This ㅡ is a weak, "empty" vowel, and it cannot survive next to the fuller vowel of an -아/어 ending. So it simply drops, and the ending's vowel takes its place in the syllable block:
쓰 + 어요 → drop ㅡ → ㅆ + 어요 → 써요
크 + 어요 → drop ㅡ → ㅋ + 어요 → 커요
For a one-syllable ㅡ stem there is no earlier vowel for the ending to harmonize with, so the ending defaults to the "dark" vowel 어 every time — never 아.
여기에 이름을 써 주세요.
yeogie ireumeul sseo juseyo
Please write your name here. (쓰다 → 써)
우리 오빠는 키가 정말 커요.
uri oppaneun kiga jeongmal keoyo
My older brother is really tall. (크다 → 커요)
나갈 때 불 좀 꺼 주세요.
nagal ttae bul jom kkeo juseyo
Please turn off the lights when you leave. (끄다 → 꺼)
The core one-syllable verbs
These few high-frequency stems cover most of what you will meet at TOPIK 1. Learn them as a set:
| Verb | Meaning | -아/어요 | Past |
|---|---|---|---|
| 쓰다 | write / use | 써요 | 썼어요 |
| 크다 | be big / tall | 커요 | 컸어요 |
| 끄다 | turn off | 꺼요 | 껐어요 |
| 뜨다 | float / open (eyes) | 떠요 | 떴어요 |
| 트다 | crack / chap / sprout | 터요 | 텄어요 |
배가 물 위에 떠 있어요.
baega mul wie tteo isseoyo
The boat is floating on the water. (뜨다 → 떠)
눈을 떠 보세요.
nuneul tteo boseyo
Try opening your eyes. (뜨다, 'open eyes' sense → 떠)
어제 친구한테 긴 편지를 썼어요.
eoje chinguhante gin pyeonjireul sseosseoyo
I wrote a long letter to my friend yesterday. (쓰다, past → 썼어요)
작년보다 키가 많이 컸어요.
jangnyeonboda kiga mani keosseoyo
I've grown a lot taller than last year. (크다, past → 컸어요)
Nothing happens before other endings
The ㅡ drops only before -아/어. Before a consonant ending, or before an 으-ending, the stem stays fully intact. And because a ㅡ stem is vowel-final (the ㅡ itself is the vowel, with no batchim), the 으-endings attach with no extra 으 — there is nothing to buffer.
| Ending | 쓰다 | Note |
|---|---|---|
| -아/어요 | 써요 | ㅡ drops |
| -았/었어요 | 썼어요 | ㅡ drops |
| -고 | 쓰고 | ㅡ kept |
| -지만 | 쓰지만 | ㅡ kept |
| -(으)면 | 쓰면 | ㅡ kept, no 으 added |
| -(으)세요 | 쓰세요 | ㅡ kept, no 으 added |
| -ㅂ니다 | 씁니다 | vowel stem → -ㅂ니다 |
텔레비전 좀 끄고 이야기해요.
tellebijeon jom kkeugo iyagihaeyo
Let's turn off the TV and talk. (끄다 → 끄고, ㅡ kept before -고)
볼펜으로 쓰면 안 돼요.
bolpeneuro sseumyeon an dwaeyo
You mustn't write in ballpoint pen. (쓰다 → 쓰면, ㅡ kept, no 으 before 면)
저는 일기를 매일 씁니다.
jeoneun ilgireul maeil sseumnida
I write in my diary every day. (formal; 쓰다 → 씁니다)
Why this isn't really "irregular"
Korean's genuinely irregular classes are lexical: some ㅂ-stems flip to 우 (돕다 → 도와) and others, spelled identically, do not (좁다 → 좁아), so you must memorize membership verb by verb. The 으 drop is nothing like that. Every ㅡ-final stem drops the ㅡ before -아/어 — there is not a single ㅡ stem that keeps it. That makes the 으 drop a phonological rule, closer to English "a → an before a vowel" than to "sing → sang." It sits in the irregular chapter only because a vowel visibly disappears; behaviorally it is as automatic as breathing.
Reframing for English speakers
English has "silent" or reduced vowels (the e in chocolate, the schwa that drops in cam(e)ra), so the idea of a weak vowel deleting is not alien. What is new is that the deletion is grammatically triggered and completely regular: the ㅡ is fine standing alone (쓰다, 크다) and fine before consonants (쓰고, 크고), but it cannot coexist with a following 아/어, so it steps aside. Don't overthink it as an "irregularity" — think of it as ㅡ being a placeholder vowel that yields the floor whenever a real vowel-initial ending shows up. The one thing to watch is multi-syllable stems, where after the ㅡ leaves you still have to pick 아 vs 어 — that lookback is the subject of the harmony page. And note that 르-stems are a special 으-drop cousin that also copies a ㄹ — a twist covered separately.
Common Mistakes
1. Keeping the ㅡ before -아/어. The ㅡ must drop; you cannot write 쓰어요.
❌ 여기에 이름을 쓰어요.
Wrong — the ㅡ of 쓰- drops before 어 → 써요.
✅ 여기에 이름을 써요.
yeogie ireumeul sseoyo
Please write your name here.
2. Failing to collapse 크다. 크 + 어요 fuses to 커요.
❌ 우리 오빠는 키가 크어요.
Wrong — the ㅡ drops → 커요, not ×크어요.
✅ 우리 오빠는 키가 커요.
uri oppaneun kiga keoyo
My older brother is tall.
3. Inserting 으 before -면. A vowel-final ㅡ stem takes plain -면, with no 으 buffer.
❌ 볼펜으로 쓰으면 안 돼요.
Wrong — no 으 on a ㅡ stem; it's 쓰면.
✅ 볼펜으로 쓰면 안 돼요.
bolpeneuro sseumyeon an dwaeyo
You mustn't write in ballpoint pen.
4. Defaulting a one-syllable ㅡ stem to 아. With no preceding vowel, the ending is always 어.
❌ 불 좀 까 주세요.
Wrong — 끄다 → 꺼 (single-syllable ㅡ always takes 어); 까 is from 까다 'to peel'.
✅ 불 좀 꺼 주세요.
bul jom kkeo juseyo
Please turn off the light.
Key Takeaways
- Any stem whose last vowel is ㅡ drops that ㅡ before an -아/어 ending: 쓰다 → 써요, 크다 → 커요, 끄다 → 꺼요.
- A one-syllable ㅡ stem has no preceding vowel, so it always defaults to 어 — never 아 (커요, not ×카요).
- The ㅡ drops only before -아/어. Before -고, -지만, -(으)면, -(으)세요 the stem is intact, and being vowel-final it takes no 으 buffer (쓰면, 쓰세요).
- Formal -ㅂ니다 attaches directly: 씁니다.
- This is a fully regular phonological rule with zero exceptions — grouped with the irregulars only because a vowel disappears.
- Multi-syllable ㅡ stems still need a 아/어 choice after the drop — see the harmony page.
Now practice Korean
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Start learning Korean→Related Topics
- 으-Drop Harmony: 바쁘다 → 바빠요 vs 예쁘다 → 예뻐요TOPIK 2 — When a multi-syllable ㅡ stem drops its ㅡ, the choice of 아 vs 어 is decided by the vowel of the syllable IMMEDIATELY before the ㅡ. ㅏ or ㅗ there → 아 (바쁘다 → 바빠요); anything else → 어 (예쁘다 → 예뻐요).
- 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.