못 vs -(으)ㄹ 수 없다: Two Ways to Say 'Can't'

Korean gives you two ways to say "can't," and they are not free variants. 못 is a short negative adverb that sits right in front of the verb: 못 가요, "I can't go." -(으)ㄹ 수 없다 is the fuller construction you already know from -(으)ㄹ 수 있다 / 없다: 갈 수 없어요, "there's no way to go." Both are grammatical, both translate as "can't," and Korean speakers switch between them constantly — but they carry different weight, and choosing the wrong one makes your Korean sound slightly off in a way that is hard to pin down. This page makes the difference explicit.

The core split: personal block vs neutral impossibility

Think of it as a difference in where the "can't" lives.

못 puts the inability on the doer. It says: something is blocking me — my body, my circumstances, a lack of skill or means. It is colloquial, immediate, and almost always about a person who wants (or is expected) to do something and can't manage it.

-(으)ㄹ 수 없다 states impossibility more neutrally and impersonally. It doesn't foreground a frustrated doer; it just reports that the thing isn't possible. It is comfortable with non-volitional subjects, abstract situations, and more formal registers.

Both are fine in both sentences, honestly — but the first leans personal and spoken, the second leans measured and explanatory. The reason 못 feels more personal is grammatical: it is an adverb glued to the verb, describing how the doer is faring, whereas 수 없다 predicates the non-existence of a possibility out in the world.

못 in action

못 is placed directly before the verb it negates. With a simple verb, that's all there is to it:

Because 못 marks inability, it naturally pairs with an external or physical obstacle:

And -(으)ㄹ 수 없다 handles the impersonal, non-volitional cases most gracefully:

The 하다-verb wrinkle

Noun + 하다 verbs (운동하다, 공부하다, 요리하다) are the one place where 못 placement trips people up. Since 못 negates the verb, careful speech splits the noun off and puts 못 in front of 하다:

In fast casual speech you will also hear 공부 못 해요 (noun kept, particle dropped) and even 못 공부해요, but the last one is dispreferred in careful Korean. The clean, always-correct pattern is [noun]을/를 못 해요 or [noun] 못 해요.

Don't confuse 못 (can't) with 안 (won't)

This is the single most consequential distinction on this page, because it changes meaning, not just tone. Korean cleanly separates:

• 못 = inability — "I can't" (something prevents me).
• 안 = choice / negation of the fact — "I don't / won't" (I'm simply not doing it).

English blurs this with "not going" — you have to infer from context whether it's unwillingness or inability. Korean forces you to decide up front, and picking 안 when you mean 못 can make you sound like you're refusing when you're actually stuck. The full contrast, including the long forms -지 않다 and -지 못하다, is on 안 vs 못 and 못: inability.

못 negates ability, not states

Because 못 specifically negates ability to do, it resists most descriptive verbs (adjectives). You can be unable to do something, but you can't be "unable to be pretty." To negate a state, use 안:

Saying ×날씨가 못 좋아요 is ungrammatical — there is no "ability" for 못 to cancel. (A handful of set expressions like 잘 못하다 "be bad at" behave differently, as we'll see next.)

A spacing trap: 못 하다 vs 못하다

Korean spelling distinguishes two things that sound nearly identical:

• 못 하다 (spaced) = "can't do it" — the ability negator, this whole page.
• 못하다 (one word) = "be bad at / do poorly" — a lexical verb about skill level.

The first says a temporary block stops you tonight; the second says singing is not your talent, full stop. Native writers keep the spacing distinct, and TOPIK graders notice.

Common Mistakes

1. Using 안 where inability (못) is meant. If something prevents you, it's 못, not 안.

2. Slotting 못 in front of the whole 하다-verb in careful speech. Split the noun off.

3. Using 못 to negate an adjective/state. States take 안, not 못.

4. Fusing 못 하다 into 못하다 when you mean "can't right now." The spacing changes the meaning.

5. Forcing 못 onto an impersonal, non-volitional subject. When circumstance itself makes something impossible, 수 없다 is more natural.

Key Takeaways

• 못 = short adverb of inability, colloquial and doer-focused (못 가요); -(으)ㄹ 수 없다 = neutral impossibility, comfortable with impersonal and formal contexts (갈 수 없어요).
• Both mean "can't," but 못 stresses "couldn't manage," 수 없다 stresses "wasn't possible."
• Never confuse 못 (can't) with 안 (won't / don't) — one is inability, the other is choice.
• With 하다-verbs, split: 공부를 못 해요, not ×못 공부해요. And keep 못 하다 ("can't") separate from 못하다 ("be bad at").