The 으-Drop: 예쁘다 → 예뻐요, 크다 → 커요

Some of the most common Korean adjectives have a stem ending in the vowel ㅡ: 예쁘다 (pretty), 바쁘다 (busy), 아프다 (sick/sore), 크다 (big), 나쁘다 (bad). When you add the -아/어요 present to these, the ㅡ vanishes — 예쁘다 becomes 예뻐요, 크다 becomes 커요, never ×예쁘어요 or ×크어요. This is the 으-drop (으 탈락), and despite the intimidating name, it is completely regular and predictable. Once you see the mechanism, every ㅡ-stem word — adjective or action verb — behaves the same way.

Why the ㅡ has to go

The ㅡ vowel is Korean's "empty" vowel — the weakest, most colorless one. When it meets the -아/어 of the present ending, Korean does not tolerate the two vowels sitting side by side (×예쁘어요). Instead the frail ㅡ simply drops out, and the ending's vowel takes its place in the syllable block. So 예쁘- + -어요 doesn't become 예쁘어요; the ㅡ deletes and the 어 slides into the 쁘 slot, giving 뻐 → 예뻐요.

This is not an exception to vowel harmony — it works with it. The only twist is figuring out which harmony vowel (아 or 어) to use, since the ㅡ that would normally decide has disappeared.

The rule in three steps

1. Drop the ㅡ.
2. Choose the harmony vowel by looking at the syllable before the ㅡ: if that syllable's vowel is ㅏ or ㅗ, use 아; otherwise use 어.
3. If there is no syllable before the ㅡ (the stem is a single ㅡ-syllable, like 크-), default to 어.

A reference table

It applies to ㅡ-stem action verbs too

The 으-drop is not an "adjective rule" — it is a stem-shape rule, and it hits ㅡ-final action verbs identically. The verb 쓰다 (to write/use) has the single-syllable stem 쓰-, so it defaults to 어 and gives 써요, exactly parallel to 크다 → 커요:

This is yet another confirmation that adjectives and verbs share one conjugation system. If you know 쓰다 → 써요, you already know 크다 → 커요; if you know 바쁘다 → 바빠요, you already know the verb 담그다 (to pickle) → 담가요. The mechanism is one and the same — see the 으-drop in verbs for the action-verb angle.

The past tense stacks predictably

Because the 으-drop happens whenever -아/어 attaches, the past tense (-았/었어요) drops the ㅡ the very same way — you just extend the vowel into the past ending. No new rule to learn:

예뻐요 → 예뻤어요, 아파요 → 아팠어요, 나빠요 → 나빴어요 — the harmony vowel you chose for the present carries straight into the past.

Don't confuse it with the 르-irregular

Here is the honest warning. A different set of stems ends in 르 (a ㄹ batchim would be different, but these end in the syllable 르): 다르다 (be different), 빠르다 (be fast), 모르다 (not know). These look like ㅡ-stems and you might expect ×다러요 by the 으-drop. But they follow a separate, irregular pattern — the 르-irregular — where the ㅡ drops and an extra ㄹ appears, doubling the ㄹ:

• 다르다 → 달라요 (not ×다러요) — "is different"
• 빠르다 → 빨라요 — "is fast"
• 모르다 → 몰라요 — "doesn't know"

There is no way to predict from spelling alone that 크다 takes the clean 으-drop (커요) while 다르다 takes the 르-doubling (달라요) — you learn which words end in 르 and flag them as irregular. The 르 family is treated in full on the 르-irregular page, and the "same/different/similar" adjectives on that page. For now, just keep them mentally separate: the 으-drop covered here is the regular one.

Common Mistakes

1. Keeping the ㅡ before the ending. The whole point is that it drops.

2. Keeping the ㅡ on a single-syllable stem.

3. Taking the harmony vowel from the wrong syllable. Look at the syllable before the ㅡ.

4. Applying the clean 으-drop to a 르-stem. 르-stems double the ㄹ instead.

Key Takeaways

• Every stem ending in ㅡ drops it before -아/어 (예쁘다 → 예뻐요, 크다 → 커요).
• Pick the harmony vowel from the syllable before the ㅡ (ㅏ/ㅗ → 아, else → 어); with no preceding syllable, default to 어 (커요, 써요).
• It is a stem-shape rule, so ㅡ-stem action verbs obey it too (쓰다 → 써요) — one system for verbs and adjectives.
• The past stacks predictably: 예뻤어요, 컸어요, 바빴어요.
• Do not confuse it with the 르-irregular (다르다 → 달라요), which looks similar but doubles the ㄹ and must be memorized as irregular.