-(으)ㄹ 줄 모르다 says something turned out other than you'd assumed: 이렇게 어려울 줄 몰랐네 'I had no idea it'd be this hard', 이렇게 성공할 사람이 많을 줄 몰랐어요 'I didn't think there'd be so many people who'd succeed'. Every line here is that surprise reading, and every one is in the past — 몰랐다 — because you're looking back at what you'd assumed. The same words in the present can also mean 'not know how to do it', but that sense does not appear in these examples.