은 줄 모르다

didn't realize it was this X

-(으)ㄴ 줄 몰랐다 expresses surprise at how much something turned out to be, and it almost always has 이렇게 "this" in front: 이렇게 어려운 줄 몰랐네..., 광수 이렇게 어깨 넓은 줄 몰랐어. In the present tense it turns into a rebuke about someone who ought to know better: 더 부끄러운 줄 모르니?, "have you no shame?" One line uses the set phrase 하늘 높은 줄 모르고, soaring without any sense of a limit. Its mirror image is -(으)ㄴ 줄 알았다, "I thought it was X" — usually wrongly.

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