-아/어야 되다 / -아/어야 하다
To say 'must' or 'have to' in Korean, take a verb's -아/어 form, add -야, then attach 되다 ('become / be okay') or 하다 ('do'). The whole thing — -아/어야 되다 / -아/어야 하다 — is one unit meaning obligation. Spoken Korean tilts toward 되다; written and slightly more formal Korean tilts toward 하다. The meaning is the same.
Form: -아/어 + 야 + 되다 / 하다
Step 1: take the -아/어 form (drop -요). Step 2: attach -야. Step 3: add 되다 or 하다, then conjugate. 가다 → 가야 되다 / 가야 하다 ('have to go'). Polite present: 가야 돼요 / 가야 해요. 먹다 → 먹어야 돼요 / 먹어야 해요. 하다 → 해야 돼요 / 해야 해요 (yes, 'have to do' = 해야 해요 — the 해 repeats).
되다 vs 하다 — same meaning, different feel
-아/어야 돼요 is what you'll hear most in conversation — softer, more colloquial. -아/어야 해요 is slightly more formal / written / emphatic. Many speakers use both interchangeably. 공부해야 돼요 and 공부해야 해요 both mean 'I have to study'. Pick 돼요 as your speaking default; recognize 해요 when reading.
Past and future obligation
Conjugate the final 되다 / 하다, not the main verb. Past = 'had to': 가야 됐어요 / 가야 했어요 = 'I had to go'. 공부해야 됐어요 = 'I had to study'. Future-intent = 'will have to': 가야 될 거예요 / 가야 할 거예요 = 'I'll have to go'. Don't put past tense on the front verb — 갔어야 돼요 is a different (counterfactual) construction for later.
Negative — what about 'don't have to'?
안 가야 돼요 is awkward — Korean expresses 'don't have to' differently. Use -지 않아도 돼요 ('it's okay not to'): 안 가도 돼요 = 'you don't have to go'. We'll formalize this contrast in a later lesson. For now: 'must' = -아/어야 돼요, 'don't have to' = 안 ___아/어도 돼요.
Compare with -고 싶다 (want)
-고 싶다 = want to (lesson 14). -아/어야 되다 = have to. The forms look similar but mean different things: 가고 싶어요 = 'I want to go'; 가야 돼요 = 'I have to go'. Common combo: 가고 싶지 않지만 가야 돼요 = 'I don't want to go but I have to'. The two stack frequently in real speech.
