In short

Two patterns for talking about knowledge. To fold a whole question inside a bigger sentence (“I don’t know whether / when / who…”), wrap the inner clause in -(으)ㄴ/는지 and follow it with 알다/모르다/궁금하다/물어보다: 언제 오는지 아세요? (Eonje oneunji aseyo?, do you know when he’s coming?). To say you know how to do something - a learned skill - use -(으)ㄹ 줄 알다/모르다: 수영할 줄 알아요 (suyeonghal jul arayo, I know how to swim). Mind the space before 줄.

The rule

Step 1 - Embed a question with -(으)ㄴ/는지

When a question becomes the object of a “knowing / asking / wondering” verb, you drop the question mark and swap the sentence-final ending for -(으)ㄴ/는지 - Korean never re-orders the words the way English does (“where is he” -> “where he is”) - then add the main verb: 알다 (alda, know), 모르다 (moreuda, not know), 궁금하다 (gunggeumhada, be curious), 물어보다 (mureoboda, ask), 기억하다 (gieokada, remember), 말하다 (malhada, tell) - and note that 기억하다’s everyday negative is the intransitive 기억이 안 나요 / 기억나지 않아요 (gieogi an nayo / gieongnaji anayo, it doesn’t come back to me), or 기억하지 못해요 (gieokaji motaeyo), because *기억하지 않아요 reads as a choice not to remember. Any question word (누가, 뭐, 언제, 어디, 왜, 어떻게, 얼마나) stays right where it sits inside the clause, and the clause’s own subject takes 이/가, never the topic 은/는 - that belongs to the main sentence (lesson 42), so it is 값이 얼마인지 몰라요, not *값은 얼마인지 몰라요:

• 누가 왔는지 몰라요. (Nuga wanneunji mollayo.) - I don’t know who came.

Pick the ending by the same verb/adjective split you use for modifier endings (lesson 24):

• Verb, present -> -는지: 오다 -> 오는지, 먹다 -> 먹는지
• Adjective, present -> -(으)ㄴ지: 크다 -> 큰지, 좋다 -> 좋은지; copula 이다 -> 인지
• Anything, past -> -았/었는지: 가다 -> 갔는지, 좋다 -> 좋았는지
• Future or still-unsettled -> -(으)ㄹ지: 가다 -> 갈지, 오다 -> 올지

Step 2 - Whether or not

To lay out both options, pair the clause with its negative, or with 아닌지 for the copula: -는지 안 -는지 (가는지 안 가는지, ganeunji an ganeunji, whether he goes or not), -(으)ㄹ지 안 -(으)ㄹ지 (갈지 안 갈지, galji an galji), -(으)ㄴ지 아닌지 (진짜인지 아닌지, jinjjainji aninji, whether it’s real or not).

Step 3 - Know how to: -(으)ㄹ 줄 알다/모르다

Verb stem + -(으)ㄹ 줄 알아요 = know how to; -(으)ㄹ 줄 몰라요 = don’t know how to. 줄 (jul) is a bound noun (a way / method), so it takes -(으)ㄹ before it and a space on each side: 할 줄 알아요, 먹을 줄 알아요, 만들 줄 알아요. This is know-how you acquired - contrast -(으)ㄹ 수 있어요 (lesson 34), which is bare possibility. They often overlap (수영할 줄 알아요 and 수영할 수 있어요 both work), but only 줄 알다 says you learned it.

Step 4 - I (wrongly) thought: -(으)ㄴ/는 줄 알았다

Switch 알다 to the past 알았다 and put a tense-matched modifier ending (-(으)ㄴ / -는 / -(으)ㄹ, not just -(으)ㄹ) in front, and the meaning flips to a mistaken assumption you held: 한국 사람인 줄 알았어요 (Hanguk saramin jul arasseoyo, I thought you were Korean - turns out not). Also 간 줄 알았어요 (I thought he’d left), 올 줄 알았어요 (I expected he’d come). The mirror image is -(으)ㄴ/는 줄 몰랐어요 = I didn’t realize that…: 오늘 시험인 줄 몰랐어요 (Oneul siheomin jul mollasseoyo, I didn’t know there was a test today).

Spacing, once more: -(으)ㄴ/는지 is one ending, no space (오는지); 줄 is a separate word, space before it (할 줄 알아요).

Forms & patterns

-(으)ㄴ/는지 - the ending by word type and tense:

-(으)ㄹ 줄 알다 - the ending picked by the 받침:

The three “knowing” frames side by side:

Examples

Common mistakes

• 좋는지 몰라요. (Jonneunji mollayo.) -> 좋은지 몰라요. (Joeunji mollayo.) - 좋다 is an adjective, so it takes -(으)ㄴ지, not the verb ending -는지.
• 어제 왔은지 몰라요. (Eoje waseunji mollayo.) -> 어제 왔는지 몰라요. (Eoje wanneunji mollayo.) - The past -았/었- is always followed by -는지 (왔는지), never *-았은지.
• 수영할 줄 있어요. (Suyeonghal jul isseoyo.) -> 수영할 줄 알아요. (Suyeonghal jul arayo.) - 줄 pairs with 알다/모르다 (know / don’t know how), never with 있다/없다. Possibility with 있다 is 수 (수영할 수 있어요).
• 만들줄 알아요. (Mandeuljul arayo.) -> 만들 줄 알아요. (Mandeul jul arayo.) - 줄 is a bound noun, so 띄어쓰기 puts a space before it: -(으)ㄹ 줄.
• 한국 사람인 줄 알아요. (meaning “I thought you were Korean”) -> 한국 사람인 줄 알았어요. (Hanguk saramin jul arasseoyo.) - The mistaken-belief reading needs the past 알았어요 - the belief is one you have since dropped. Present 알아요 does not mean you know it: with a -(으)ㄴ/는 modifier, 줄 알다 is always a supposition, so it says you still hold the assumption (I take you for a Korean). For actual knowledge use -(으)ㄴ/는 것을 알다.

Tips

• 는 for actions, (으)ㄴ for states. In the present, verbs take -는지 (움직임: 오는지, 먹는지) and adjectives/copula take -(으)ㄴ지 (상태: 좋은지, 인지). The past forgets the split entirely - everything becomes -았/었는지.
• -는지 (now) vs -(으)ㄹ지 (unsettled). 오는지 아세요? asks whether he is coming; 올지 아세요? asks whether he will come. Reach for -(으)ㄹ지 whenever the fact is still open or you’re guessing about the future.
• One word 줄, two jobs. Present 알다/모르다 = a learned skill (할 줄 알아요, know how); past 알았다 = a wrong assumption (인 줄 알았어요, I thought…). Same 줄, the tense on 알다 tells you which.
• 줄 알다 vs 수 있다. 줄 알다 = you learned it; 수 있다 = it’s possible now. A pianist with a broken arm still 칠 줄 알아요 (has the know-how) even though today she 못 쳐요 (can’t play).