줄: -(으)ㄹ 줄 알다 / 모르다 (know how / know or think that)

The bound noun 줄 is one of those small words that looks like a single grammar point but is secretly two. Always paired with 알다 (to know) or 모르다 (to not know), it packs two meanings that English keeps apart with entirely different verbs: knowing *how to do something (a skill) and knowing — or wrongly assuming — **that something is the case* (a belief). Both readings sit on the same modifier endings you already use with 것 and 수, so once you can attach -는 / -(으)ㄴ / -(으)ㄹ to a clause, you can build 줄. The trick is telling the two 줄s apart — and, above all, not confusing either of them with 수 있다.

(One quick disambiguation: this grammatical 줄 has nothing to do with the everyday noun 줄 meaning "line, rope, queue." Same spelling, unrelated word. Here it is a bound noun — it cannot stand alone and must lean on a modifier clause in front of it.)

Sense 1 — 알다: "know how to" (skill / method)

With the prospective modifier -(으)ㄹ, the frame -(으)ㄹ 줄 알다 / 모르다 means to know / not know how to do something — to possess or lack the know-how, the learned method.

Notice that this "skill" sense only ever uses -(으)ㄹ 줄 — the prospective form. You cannot say ×수영하는 줄 알다 to mean "know how to swim." The -는 / -(으)ㄴ modifiers belong to the other 줄, coming up next.

The reframing that matters most: 줄 알다 is NOT 수 있다

English hides a distinction that Korean forces you to make. "I can swim" can mean two things:

1. I have the skill — I learned how; I'm not a non-swimmer. → 수영할 줄 알아요
2. I am physically able / permitted right now — nothing is stopping me. → 수영할 수 있어요

Most of the time they overlap, but pull them apart and the difference is sharp. A trained swimmer with a broken arm still knows how to swim (줄 알다) but cannot swim today (수 없다). This single sentence shows the whole contrast:

Because of this, temporary inability is 수 없다, never 줄 모르다 — 줄 모르다 says you never learned in the first place.

Sense 2 — 알다 / 모르다: "know / think that" (belief about a fact)

Swap the modifier and 줄 changes jobs. With -는 / -(으)ㄴ / -(으)ㄹ 줄 알다 / 모르다, 줄 marks a fact you know, or believe, or fail to realize. Here 알다 means "be aware / assume" and 모르다 means "not realize."

The modifier does the tense-and-word-class work, exactly as it does for relative clauses. Here is the full set:

The famous twist: past 줄 알았다 often means "wrongly assumed"

This is where 줄 earns its reputation. Present-tense 줄 알다 ("I'm aware that...") is straightforward. But the past form 줄 알았다 carries a strong pragmatic tilt: much of the time it means "I assumed X — and I was wrong." The very act of reporting your past belief implies reality contradicted it.

Say 온 줄 알았어요 and a Korean listener will almost always hear "I thought he'd come — but he hadn't." This mistaken-assumption flavor is baked in, and English speakers routinely miss it, producing sentences that sound accidentally accusatory or simply confusing.

That said, 줄 알았다 is genuinely ambiguous: with the prospective -(으)ㄹ 줄 알았다, it can just as easily mean "I knew it would happen" — a correct prediction.

How do you tell the two apart? Context, and often a following -는데 or 그런데: 올 줄 알았는데 안 왔어요 ("I expected him to come, but he didn't"). When in doubt, the fact-sense past (-(으)ㄴ 줄 알았다, "I thought X had happened") leans strongly toward wrong, while the prospective past (-(으)ㄹ 줄 알았다, "I knew X would happen") can be either — the sentence usually spells out which.

Common Mistakes

1. Using 줄 알다 for physical ability or permission — that's 수 있다. 줄 알다 is a skill. A one-off possibility (being able or allowed today) is 수 있다.

• ✗ 오늘은 다리를 다쳐서 걸을 줄 몰라요. (sounds like you never learned to walk)
• ✓ 오늘은 다리를 다쳐서 걸을 수 없어요.

2. Missing the "wrong assumption" nuance of past 줄 알았다. Translating 온 줄 알았어요 as a flat "I knew he came" misfires — it means "I thought he came (but he didn't)." If you actually knew and were right about a past event, use plain 알고 있었어요: 온 걸 알고 있었어요.

3. Wrong modifier for the word class — especially with 이다. The copula's present modifier is -인, not -이는.

• ✗ 그 사람이 학생이는 줄 알았어요.
• ✓ 그 사람이 학생인 줄 알았어요.

4. Attaching 줄 straight to a noun or bare stem. 줄 is a bound noun: it needs a full modifier clause in front, never a bare noun or stem.

• ✗ 저는 수영 줄 알아요. / ✗ 저는 수영하 줄 알아요.
• ✓ 저는 수영할 줄 알아요.

5. Using -는/-(으)ㄴ 줄 for the skill sense. "Know how to" is locked to -(으)ㄹ 줄. ×운전하는 줄 알아요 does not mean "I know how to drive" (it would mean "I think someone is driving"). For the skill, it must be 운전할 줄 알아요.

Key Takeaways

• -(으)ㄹ 줄 알다/모르다 = know how to (skill). It is not 수 있다 (ability/permission): a broken arm kills the 수 있다, not the 줄 알다.
• -는 / -(으)ㄴ / -(으)ㄹ 줄 알다/모르다 = know / think / fail to realize that (belief), with the modifier carrying tense and word class.
• Past 줄 알았다 usually signals a mistaken assumption in the fact sense (온 줄 알았어요 = "I thought he came, but…"), while prospective 될 줄 알았다 can be a correct prediction.
• The copula takes -인 줄 (학생인 줄), never -이는 줄.