수: Ability & Possibility with -(으)ㄹ 수 있다 / 없다

If you learn one way to say "can" in Korean, learn this one. The bound noun 수 — a word that means something like "way / means / possibility" — is almost frozen into a single construction: -(으)ㄹ 수 있다 ("can") and -(으)ㄹ 수 없다 ("cannot"). 할 수 있어요 is "I can do it"; 갈 수 없어요 is "I can't go." It is one of the highest-frequency grammar points in the whole language, and it hides a beautifully logical structure that, once you see it, makes the negation rule impossible to forget.

The literal logic: "there is a way to…"

English "can" is a single modal verb — one word that carries both the meaning and its own negation (can / cannot). Korean does it completely differently. -(으)ㄹ 수 있다 is not a modal at all; it's an existential statement. Break it apart:

• -(으)ㄹ 수 — literally "a way / means to VERB" (수 is the noun "way," 할 수 = "a way to do")
• 있다 — "there is / exists"
• 없다 — "there is not / does not exist"

So 할 수 있어요 is, word for word, "a way to do it exists" = "I can." And 할 수 없어요 is "a way to do it does not exist" = "I can't." The polarity — can versus cannot — lives entirely in the choice between 있다 (exists) and 없다 (doesn't exist). This is why you never negate this construction with 안 or 못: there is nothing for them to negate. You just swap the existence verb.

Attaching -(으)ㄹ 수: the allomorphy

The construction hangs on the prospective modifier -(으)ㄹ, the same ending you met with 것. Which shape you use depends on the verb stem:

That third row matters. A ㄹ-final stem already ends in ㄹ, so the -(으)ㄹ ending simply merges into it — you write a single ㄹ, never a doubled one and never an added 을. 살다 → 살 수 있어요, not ×살을 수; 만들다 → 만들 수 있어요, not ×만들을 수.

Irregular stems reshape before the ending in their usual way — a ㅂ-irregular like 돕다 becomes 도울 수, a ㄷ-irregular like 듣다 becomes 들을 수:

Where the tense goes: on 있다 / 없다

To put the "can" into the past, you do not touch the main verb — the modifier stays -(으)ㄹ. The tense goes on the existence verb: 있다 → 있었다, 없다 → 없었다.

Say 갈 수 있었어요 for "I was able to go," not ×갔을 수 있어요 — the latter puts past tense on the wrong verb and derails the meaning entirely.

A note on pronunciation vs. spelling

Two sounds shift here, and both stay hidden in the spelling. The 수 after -(으)ㄹ is pronounced tense, [쑤] — 할 수 comes out [할 쑤] — and 있다 is pronounced [읻따]. But you always write 할 수 있다, with the plain 수 and the plain 있다. Don't let the tense sound tempt you into ×할 쑤 or ×할 수 잇다.

수밖에 없다: "have no choice but to"

수's one other frozen home is -(으)ㄹ 수밖에 없다 — literally "there is no way other than to VERB," i.e., "have no choice but to." It reuses the same "way exists / doesn't exist" logic, with 밖에 ("outside of / except") narrowing it to a single option.

Not the only "can": 못 and 줄 알다

-(으)ㄹ 수 있다 covers most of English "can," but Korean has two neighbors it should not be confused with. 못 / -지 못하다 focuses on inability — being blocked from doing something — and often competes with 수 없다 (compare them on 못 vs. -(으)ㄹ 수 없다). And -(으)ㄹ 줄 알다 means "know how to" — a learned skill — which is a different idea from "be able to right now." A swimmer with a broken arm still knows how to swim (수영할 줄 알다) but cannot swim today (수영할 수 없다); see -(으)ㄹ 줄 알다.

Common Mistakes

1. Wrong allomorph — treating 하다 like a consonant stem. 하다 ends in a vowel, so it takes -ㄹ 수. The same for 가다.

• ✗ 하을 수 있어요 / ✗ 가을 수 있어요
• ✓ 할 수 있어요 / ✓ 갈 수 있어요

2. Negating with 안 or 못 instead of 없다. "Cannot" is 수 없다, full stop — there is no negator inside this construction.

• ✗ 안 할 수 있어요 / ✗ 못 갈 수 있어요
• ✓ 할 수 없어요 / ✓ 갈 수 없어요

3. Doubling the ㄹ on a ㄹ-stem. A stem already ending in ㄹ absorbs the -(으)ㄹ — no added 을.

• ✗ 만들을 수 있어요 / ✗ 살을 수 있어요
• ✓ 만들 수 있어요 / ✓ 살 수 있어요

4. Putting the past tense on the main verb. The modifier stays -(으)ㄹ; the past goes on 있다/없다.

• ✗ 갔을 수 있어요 (for "I was able to go")
• ✓ 갈 수 있었어요

5. Gluing 수 to the verb with no space. 수 is a bound noun but still a separate word — leave a space before it.

• ✗ 할수 있어요
• ✓ 할 수 있어요

Key Takeaways

• -(으)ㄹ 수 있다 / 없다 = "can / cannot," literally "a way to VERB exists / doesn't exist."
• Polarity is carried by 있다 (can) vs. 없다 (cannot) — never by 안 or 못.
• Allomorph: vowel stem → -ㄹ 수 (갈 수), consonant stem → -을 수 (먹을 수), ㄹ-stem → single ㄹ (살 수, 만들 수).
• Tense goes on 있다 / 없다 (갈 수 있었어요), not on the main verb.
• Pronounced tense ([쑤], [읻따]) but always spelled 할 수 있다.
• Distinct from 못 / -지 못하다 (inability) and -(으)ㄹ 줄 알다 (know how to).