ShadowingKorean

-(으)ㄹ 수 있다, -고 싶다, -아/어야 되다 — can, want and must in Korean

Five endings that arrive at the same moment: can, want to, have to, may, and must not. Plus the one contrast English speakers get wrong for years.

Ability — -(으)ㄹ 수 있다 / 없다, and 못

Attach -을 수 있다 after a consonant and -ㄹ 수 있다 after a vowel. The negative swaps 있다 for 없다.

먹다 → 먹을 수 있어요.I can eat
가다 → 갈 수 있어요.I can go
갈 수 없어요.I can't go
못 가요.I can't go — the everyday spoken version

못 and -(으)ㄹ 수 없다 overlap heavily; don't hunt for a rule that separates them cleanly. The usable difference is register: 못 is short and spoken, -(으)ㄹ 수 없다 sounds more categorical, closer to it is not possible.

With 하다 verbs, 못 goes inside, exactly like 안 — and the spacing carries meaning.

공부 못 해요.can't study — something is stopping me
공부못해요.is bad at studying — written together, 못하다 is its own verb

Want — -고 싶다, and why you can't use it about other people

-고 싶다 on the verb stem. No vowel harmony, no changes — it is the easiest ending in this article.

가고 싶어요.I want to go
뭐 하고 싶어요?What do you want to do? — fine, you're asking the listener
동생이 가고 싶어해요.My sibling wants to go
동생이 가고 싶어요.✗

The rule: -고 싶다 for yourself and for questions to the person in front of you; -고 싶어하다 when you report a third person's wish. Korean treats someone else's inner state as something you observe, not something you know.

You will hear the object marked either way — 물을 마시고 싶어요 and 물이 마시고 싶어요 are both used. Neither is a mistake.

Must, may, must not — and the one that trips everyone

These build on the 아/어 form you already make for 아요/어요, so if you can say 먹어요 you can say 먹어야 돼요.

가야 돼요. / 가야 해요.I have to go — 돼요 in speech, 해요 a shade more formal
가도 돼요?May I go?
가면 안 돼요.You must not go
안 가도 돼요.You don't have to go

Look at the last two. “Must not” and “don't have to” are not variations of each other in Korean — 가면 안 돼요 forbids, 안 가도 돼요 releases. English hides this behind the same word must, and learners reach for 가지 않아야 돼요, which nobody says.

One more, because it is the other half of “don’t”: a negative command uses neither 안 nor -지 않다. It uses -지 마세요. There is no way to build it out of the forms above — 안 가세요 means you don’t go, said politely, not don’t go.

가지 마세요.Don't go
걱정하지 마세요.Don't worry
가지 마.Don't go — 반말
가면 안 돼요. / 가지 마세요.“You must not go” states a rule; “don't go” gives an order

Asking for something — -아/어 주세요

Not a modal, but it belongs here: it is the sixth thing you need the moment you can say the other five.

도와주세요.Please help me
잠깐 기다려 주세요.Please wait a moment
물 주세요.Water, please — a bare noun takes 주세요 on its own
다시 말해 주시겠어요?Could you say that again? — softer, more polite

Where they sit — before tense, around honorifics

All of these are frames that wrap the verb, and tense goes on the outside. The past tense marks 싶다 or 되다, never the main verb.

가고 싶었어요.I wanted to go
갔고 싶어요.✗ — the past belongs on 싶다
갈 수 있었어요.I was able to go
가야 했어요.I had to go
가실 수 있어요?Can you go? — honorific -시- goes inside, on the main verb

This is the same layering described in how Korean verbs are built: stem, then honorific, then tense, then the ending. These frames simply add one more layer at the point where the ending goes.

Five mistakes worth memorising

The sets below are spoken sentences, which is where these endings actually live — 가도 돼요? and 가야 돼요? are things you hear far more often than you read.

One honest qualification. 수 있다 is not sealed off from permission. In a shop or someone’s home, 여기서 사진 찍을 수 있어요? is heard as asking whether photos are allowed — in that setting “is it possible” and “am I allowed” collapse into each other. 어도 돼요 always lands as permission; 수 있다 only sometimes does. Use the one that never has to be guessed at.

Practice this

All guides