ㅎ-irregular and ㅡ-irregular
Korean has a handful of regular-looking verb stems that behave irregularly when you attach -아/어요. Today we cover two of the most common: ㅎ-irregular (mostly color adjectives like 빨갛다) and ㅡ-irregular (verbs whose stem ends in ㅡ, like 바쁘다). Both have one simple trick — you just need to recognize when it triggers.
ㅎ-irregular: drop ㅎ, fuse the vowel
Most adjectives ending in -ㅎ다 are irregular. When -아/어요 attaches, the ㅎ drops AND the stem vowel fuses with -아/어 into -애요 (or -얘요 for 이렇다 / 그렇다 / 저렇다 / 어떻다). 빨갛다 (red) → 빨갛- + -아요 → 빨개요. 노랗다 (yellow) → 노래요. 하얗다 (white) → 하얘요. 그렇다 (be like that) → 그래요. The plain past tense follows the same fusion: 빨갰어요, 그랬어요. Note: 좋다 (good) and 놓다 (put) are NOT ㅎ-irregular — they're regular: 좋아요, 놓아요.
Which adjectives are ㅎ-irregular?
Almost every color word: 빨갛다 (red), 파랗다 (blue), 노랗다 (yellow), 하얗다 (white), 까맣다 (black). Plus the four demonstratives 이렇다 / 그렇다 / 저렇다 / 어떻다 ('like this / that / over there / how'). When you meet a new -ㅎ다 adjective, assume it's irregular unless told otherwise.
ㅡ-irregular: drop ㅡ, then pick -아/어
When a verb stem ends in the vowel ㅡ and you attach -아/어요, the ㅡ simply drops. Then you choose -아 or -어 based on the previous syllable's vowel: ㅏ/ㅗ → -아, anything else → -어. 바쁘다 (busy) → 바쁘- → 바ㅃ- → vowel before is ㅏ, so 바빠요. 예쁘다 (pretty) → 예쁘- → 예ㅃ- → vowel before is ㅖ (not ㅏ/ㅗ), so 예뻐요. 슬프다 (sad) → 슬ㅍ- → ㅡ, so 슬퍼요. 크다 (big) → 크- → no preceding syllable, default to -어 → 커요. 쓰다 (write) → 써요.
Past tense, same trick
The past -았/었어요 uses the exact same rule: drop the ㅡ first, then pick -았 or -었. 바쁘다 → 바빴어요 (was busy). 예쁘다 → 예뻤어요. 크다 → 컸어요. The trick only fires before vowel-initial endings like -아/어 — before consonant-initial endings like -고, -지만, the ㅡ stays: 바쁘고, 예쁘지만.
Quick contrast
ㅎ-irreg is a consonant dropping (the ㅎ) plus vowel fusion. ㅡ-irreg is a vowel dropping (the ㅡ) with normal -아/어 selection. Both rules only kick in before -아/어 endings. Both are extremely common — color adjectives alone are everywhere, and 바쁘다 / 예쁘다 / 아프다 / 기쁘다 are daily vocabulary.
