걸 모르다 / 는지 모르다 / 줄 모르다 They are sometimes interchangeable but there are also many situations where one is better suited than the others. * 줄 is a word made for 알다/모르다 and used widely in common, everyday situation (colloquial). In particular, 1) -ㄹ 줄 알다/모르다 (note ㄹ, not ㄴ) can mean "know how to do something", as in 나 수영할 줄 알아. 2) 줄 알다 can also mean "think mistakenly", which sets it apart from the rest. * 는지 means "if" or "whether", so it's well suited to judgments, decisions, and when speaking of a degree or extent. 1) true-false decisions and when there's a question word like how, why, when, etc. 2) -ㄴ/은/는지 also makes the expression "not sure of something", as in 그럴지(도)/그런지 몰라. * 걸 is based on 것 which means a thing or a fact, so it works best when referring to a concrete fact than a judgment. 1) 걸 is the best choice when there's a quoting expression like -(이)라는 or -다는. 2) 걸 cannot be used with questions words at all. Examples: - 나는 자전거 탈 줄 알아. I know how to ride a bicycle. - 나는 네가 *결혼 한 줄* 알았어. I thought you got married. (not true). - 나는 네가 결혼 한 줄 *알았어*. I already knew you got married. (true) = 나는 네가 결혼한 걸 알았어(clearer) - 넌 내가 얼마나 널 좋아하는지 몰라. You don't know how much I like you. (줄 Ok, 걸 bad) - 어찌 해야 할지 모르겠다. I don't know what I should do. (줄 slightly unnatural, 걸 bad) - 그럴지도 모르지. That may be so (줄 bad, 걸 bad) - 이렇게 쉬운 걸 몰라서 고생했다: I had a hard time confused with such a simple thing (는지 bad, 줄 changes meaning) (when -(이)라는 or -다는 is involved) 1 어제가 마감 날이란 걸 알았다: I knew YESTERDAY WAS THE DEADLINE (natural). 2 어제가 마감 날이란 줄 알았다: I thought THEY SAID yesterday was the deadline (different meaning) 3 어제가 마감 날이라는지 알았다: (barely Ok with the same meaning as 줄) * #1 is equivalent to '마감 날이다'라는 걸(fact), #2 and #3 to (누군가가) 마감 날이라고 말하는 줄 [말하는지] 알았다 (quote).

걸 모르다 / 는지 모르다 / 줄 모르다 1. 걸 모르다 is short for 것을 모르다 2. 는지 모르다 is more related to probability 3. 줄 모르다 is short for 지를 모르다 1,2 and 3 are very similar, but 1 and 3 are more similar to each other. For example, - She doesn't know that I'm eating her cake now. 1. 그녀는 내가 지금 그녀의 케익을 먹는 중인걸 모른다. 2. 그녀는 내가 지금 그녀의 케익을 먹는 중인지 모른다. 3. 그녀는 내가 지금 그녀의 케익을 먹는 중인줄 모른다. These three sentences are saying same content. But the second one can be more similar to 'She doesn't know whether or not I'm eating her cake now.' in some contexts. To be a more distinctive sentence, it will be spoken as '그녀는 내가 지금 그녀의 케익을 먹는 중인지 아닌지 모른다.' In this case, listeners may think she might expect that he would eat her cake. In the other cases, all of those sentences are completely same. Just you have to be careful for the usage of the second one, because it will mean 'whether or not' in more cases.

모르다 mean do not know. 는지 모르다 : do not know what something was happened. ex) 맞는지 모르다. I don't know it was right. 걸 모르다 = 줄 모르다 : do not know what something available. ex) 말하는 걸 모르다. 말할 줄 모르다. I don't know how to speak. *걸 = 것을 알다 mean do know. and same as above.