-(으)ㄹ 수 있다 / 없다: Can / Cannot

If you learn only one way to say "can" in Korean, this is the one. -(으)ㄹ 수 있다 ("can / be able to") and its negative -(으)ㄹ 수 없다 ("cannot") are the workhorse ability construction — grammatical, neutral, and correct in almost any situation, from ordering coffee to writing a report. What makes it feel foreign at first is that it is not a single modal verb like English can. It is built from a little machine of three parts, and once you can see the parts, the whole thing becomes predictable.

What the pieces are doing

The 수 in the middle is a bound noun — a noun that cannot stand alone and only appears propped up by a modifier in front of it. Its lexical meaning is roughly "way, means, method, possibility." So a sentence like 갈 수 있어요 is literally "there exists a way to go," and 갈 수 없어요 is "there is no way to go." The 있다/없다 on the end are the ordinary existence verbs "there is / there isn't." Korean expresses ability the way English might say there's a way I can do it — as the existence or non-existence of a possibility.

That analysis is not just trivia; it explains the shape. The verb in front has to be a modifier, not a plain stem, which is why you attach the prospective modifier ending -(으)ㄹ: 가다 → 갈, 먹다 → 먹을, 읽다 → 읽을. And because 수 is a separate noun, it is written with spaces on both sides: 갈 수 있어요, never ×갈수있어요.

Forming it: which vowel goes on the verb

The only fiddly part is the modifier ending, and it follows the same batchim rule you already use everywhere else:

The ㄹ-stem row is the one that catches people: because the stem already ends in ㄹ, you do not add 을. 만들다 ("to make") becomes 만들 수, never ×만들을 수. For the full logic of why ㄹ-final stems behave this way, see ㄹ-irregular verbs.

A note on pronunciation: 수 있다 comes out as [쑤 읻따] — the 수 is tensed to [쑤] and 있다 to [읻따]. That tensing is real, but Revised Romanization does not write it, so the reading aid shows su isseoyo, not "ssu itta." Just know that if it sounds like your teacher is saying "ssu," your ears are right.

One form, two jobs: ability and possibility

Here is where English speakers are actually well served by their intuitions. Just like English can, -(으)ㄹ 수 있다 spans two meanings that Korean does not bother to distinguish:

Learned ability — a skill or capacity the subject has:

Situational possibility — the circumstances allow it, regardless of anyone's skill:

That last pair is pure circumstance — the app makes it possible, the rule makes it impossible; nobody's personal ability is in question. Because the construction is built on "there exists / does not exist a way," it slides between these readings with no change in form. This is exactly the kind of overlap that a language like French splits (savoir vs pouvoir) but English and Korean happen to leave fused.

It also inflects for tense on the 있다/없다, exactly like any existence verb:

The big trap: "cannot" is not "no way that"

This is the distinction that separates a careful learner from a sloppy one. Both of the following English-sounding ideas involve "can't," but Korean uses two different constructions:

• -(으)ㄹ 수 없다 = "unable to / there is no way for me to do it." It is about capacity or possibility.
• -(으)ㄹ 리가 없다 = "there's no way (that) / it can't be the case that." It is a confident inference about what is or isn't true.

Compare:

Swap them and you get nonsense: 민수가 그런 말을 할 수 없어요 would mean Minsu is incapable of physically producing those words, which is not what an indignant friend means. When you want to say "no way — I don't believe it," reach for 리가 없다, covered on its own page: -(으)ㄹ 리가 없다: no way / it can't be.

Where this sits among its neighbors

-(으)ㄹ 수 있다/없다 is the general-purpose center of a small family, and it is worth knowing the two forms that carve off narrower slices of its meaning:

• 못 is a short, colloquial adverb of inability — 못 가요 ("I can't go") often lands more personally than 갈 수 없어요. When to prefer which is the whole subject of 못 vs -(으)ㄹ 수 없다.
• -(으)ㄹ 줄 알다 narrows to know-how — a learned skill specifically, not situational possibility. "I know how to swim" vs "I'm able to swim right now" is the contrast handled on -(으)ㄹ 줄 알다 / 모르다.

For the deep dive on 수 as a bound noun in its own right, see the bound noun 수.

Common Mistakes

1. Dropping the spaces (or writing it as one word). 수 is a noun and must be spaced off on both sides. This is a spelling error even native texters avoid.

2. Adding 을 to a ㄹ-stem. ㄹ-final stems already supply the modifier ㄹ; there is no extra 을.

3. Using the dictionary form instead of the -(으)ㄹ modifier. The verb in front must carry -(으)ㄹ, never the bare -다 form.

4. Using 수 없다 for a confident inference. "No way he'd do that" is disbelief, not inability.

5. Reaching for 알다 ("know") to mean "can." English can swim tempts learners into 수영을 알아요, but general ability is 수 있다; know-how specifically is 줄 알다.

Key Takeaways

• -(으)ㄹ 수 있다 / 없다 literally says "there is / isn't a way to _" — a bound noun 수 between a -(으)ㄹ modifier and 있다/없다.
• It covers both learned ability and situational possibility, like English can — no need to split them.
• Batchim → 을 수 (먹을 수), vowel or ㄹ-stem → ㄹ 수 (갈 수, 만들 수). Always spaced.
• Don't confuse 수 없다 ("unable") with 리가 없다 ("no way that / it can't be true"). One is about capacity, the other about belief.