은 줄 알다

I thought so — and was wrong

줄 알다 is believing something to be the case, and it nearly always turns out to have been mistaken: 눈이 되게 좋은 줄 알았어요 "I thought you had great eyesight", 차려입은 줄 알았더니 "I thought you'd actually dressed up". Turned into a question aimed at someone it becomes a rebuke — 쉬운 줄 알아요? "you think that's easy?". The -ㄴ/-은 form points at a finished action or a quality; use -는 for something still in progress.

Register: casual spoken