-(으)ㄹ 수 있다: Can, Cannot, and the Overlap with 못

-(으)ㄹ 수 있다 says something is possible. -(으)ㄹ 수 없다 says it is not.

It is the neutral way to talk about ability, and it is worth learning alongside 못 rather than instead of it.

Building it

Add the modifier ending to the stem, then 수 있다 or 수 없다.

Vowel → -ㄹ 수 있다
Consonant → -을 수 있다
ㄹ → -ㄹ 수 있다, and the ㄹ stays

가다    →  갈 수 있어요       I can go
먹다    →  먹을 수 있어요     I can eat
만들다  →  만들 수 있어요     I can make it     ← ㄹ stays
수영하다 → 수영할 수 있어요

That ㄹ behaviour is worth noting. It drops before -(스)ㅂ니다 (삽니다) and -(으)니까 (사니까), but not here. Each ending carries its own rule.

수 on its own is a bound noun meaning a way, a means. It never appears alone, which is why the whole thing is learned as one unit rather than assembled from parts.

The overlap with 못

These two say nearly the same thing:

갈 수 없어요.
못 가요.

Both mean I can’t go. The difference is small but real:

Which matters most when you are turning something down:

A: 내일 시간 있어요? — Free tomorrow?
B: 아, 못 가요. 미안해요. ✅ warm
B: 아, 갈 수 없어요. △ sounds like a policy

못 lets the reason stay outside you and unstated. 없어요 states impossibility, which lands harder than most people intend.

For a factual impossibility, the reverse holds:

여기서는 담배를 피울 수 없습니다. — Smoking is not permitted here.

That is a sign, not a person, and 못 would be wrong for it.

Asking for things

This is where you will use the positive form most:

도와줄 수 있어요? — Could you help me?
혹시 사진 좀 찍어 주실 수 있어요? — Could you take a photo, by any chance?

Asking whether something is possible for the listener is the polite way to request. Adding 혹시 (“by any chance”) in front softens it further, and you will hear it constantly in front of requests.

Past and future

• 갈 수 있었어요 — I could go / I was able to
• 갈 수 있을 거예요 — I’ll be able to go

어릴 때 수영할 수 있었어요. — I could swim when I was young.

Quick reference