A2
아/어야 되다 — must
How Korean expresses obligation with 아/어야 되다 and 아/어야 하다, and the separate construction used for not having to.
Take the 아/어 form of the verb, add 야 되다. The vowel rule is the same one that builds the 해요 form.
지금 가야 돼요.
Jigeum gaya dwaeyo.
I have to go now.
약을 먹어야 돼요.
Yageul meogeoya dwaeyo.
I have to take my medicine.
되다 or 하다
아야 하다 means the same thing. 되다 is more common in speech, 하다 slightly more formal and more written. Neither is more polite.
숙제를 해야 합니다.
Sukjereul haeya hamnida.
Homework must be done.
Not having to is a different construction
There is no negative of this form. Not having to is 지 않아도 되다 — it is fine even if you don’t.
안 와도 돼요.
An wado dwaeyo.
You don’t have to come.
가야 안 돼요.
안 가도 돼요.
가야 안 돼요 is not a sentence. This form has no negative: needn’t is 안 가도 돼요, while the construction that does negate 되다 — 가면 안 돼요 — means mustn’t, which is the opposite of what was meant.
That contrast is worth holding onto: 안 가도 돼요 is you needn’t, 가면 안 돼요 is you mustn’t. See permission.
In speech it shortens
돼요 becomes 되 in casual speech, and the whole thing often collapses: 가야 돼 → 가야지, or just 가야겠다 when you are talking yourself into it.
이제 가야겠어요.
Ije gayagesseoyo.
I’d better get going.