ㅂ-irregular: ㅂ → 우 before vowel-initial endings
Many descriptive verbs (the ones that translate as English adjectives) end in ㅂ: 춥다 (be cold), 덥다 (be hot), 쉽다 (be easy), 어렵다 (be hard), 맵다 (be spicy), 무겁다 (be heavy), 가볍다 (be light). When you attach a vowel-starting ending (like -아요/어요, -(으)니까, -(으)면), the ㅂ changes to 우 and merges with the following vowel as 워: 춥다 → 추워요 (it's cold). The ㅂ stays put before consonant endings — 춥지만, 춥고 are normal. This pattern hits the majority of descriptive ㅂ-final verbs and is the second most-common irregular paradigm after ㄹ.
The basic transformation: ㅂ → 우 → 워
Take 춥다 (be cold). Drop -다 to get the stem 춥-. The polite present ending is -어요, which starts with the vowel ㅓ. The irregularity says: replace ㅂ with 우, then attach -어요. So 춥 + 어요 → 추우 + 어요 → contracts to 추워요. Same pattern for the others: 덥다 → 더워요 (hot), 쉽다 → 쉬워요 (easy), 어렵다 → 어려워요 (difficult), 맵다 → 매워요 (spicy), 무겁다 → 무거워요 (heavy), 가볍다 → 가벼워요 (light), 귀엽다 → 귀여워요 (cute), 즐겁다 → 즐거워요 (fun/enjoyable). The change happens consistently before any vowel-initial ending.
Two exceptions: 돕다 and 곱다 → ㅂ becomes 오, not 우
There are exactly TWO ㅂ-irregular verbs where ㅂ becomes 오 instead of 우: 돕다 (to help) → 도와요 (NOT 도워요), and 곱다 (to be pretty/elegant — somewhat archaic) → 고와요. Past: 도왔어요, 고왔어요. The rule of thumb in modern grammar books: if the stem has the vowel ㅗ (only 돕 and 곱), use 오; everything else uses 우. So you'll see 도와주세요 ('please help' — the standard polite request from lesson 19), where 돕 + -아요 → 도와요. Apart from these two, every other ㅂ-irregular verb uses 우 → 워.
Past tense: -ㅂ → -웠어요 (or -왔어요)
The past works the same way. 춥다 + -었어요 → 추웠어요 (it was cold). 덥다 + -었어요 → 더웠어요 (it was hot). 어렵다 + -었어요 → 어려웠어요 (it was hard). 맵다 + -었어요 → 매웠어요 (it was spicy). For the two 오-exceptions: 돕다 + -았어요 → 도왔어요 (helped), 곱다 + -았어요 → 고왔어요 (was pretty). Notice in past tense the -아요/-어요 choice for 돕다/곱다 is -았어요 (because the vowel becomes ㅗ, an ㅗ/ㅏ vowel that pairs with -았-).
Before consonant endings: ㅂ stays
The irregularity ONLY kicks in before vowel-starting endings. Consonant-starting endings keep the ㅂ. 춥다 + -지만 → 춥지만 (it's cold but…). 춥다 + -고 → 춥고 (it's cold and…). 어렵다 + -지만 → 어렵지만 (it's hard but…). 맵다 + -지만 → 맵지만 (it's spicy but…). For -(으)면, the ending starts with 으 (a vowel), so the ㅂ-irregular DOES kick in: 춥다 + -(으)면 → 추우면 (if it's cold). 어렵다 + -(으)면 → 어려우면. Same with -(으)ㄹ 수 있다: 어려울 수 있어요 (it might be hard).
Regular ㅂ-stems exist too
Not every verb ending in ㅂ is irregular. A small set of action verbs are regular and keep their ㅂ everywhere: 잡다 (to grab/catch) → 잡아요 (NOT 자워요), 씹다 (to chew) → 씹어요, 입다 (to wear) → 입어요, 좁다 (to be narrow) → 좁아요. The general guideline: most DESCRIPTIVE verbs in ㅂ are irregular (the 'feels like an adjective' ones), while a handful of ACTION verbs in ㅂ stay regular. You'll need to memorize the regular set as you meet them, but if a ㅂ-final verb describes a STATE (cold, hot, easy, heavy, cute, spicy), the safe default is to assume it's ㅂ-irregular.
