‘-은/ㄴ 줄 알다, -는 줄 알다, -인 줄 알다’ is used to say you expect something is being done, but it isn’t. When For an adjective, it means “I expected SUBJECT is (adjective) [but SUBJECT is not].” When the expectation is of the past, use “-은/ㄴ 줄 알다“. When the expectation is of the future, use “-을/ㄹ 줄 알다“.
● 오늘 비가 오는 줄 알았어요. (그런데 안 왔어요.) : I expected it will rain today. (But I was wrong.)
● 시험이 쉬울 줄 알았어요. (그런데 어려웠어요.): I expected the exam would be easy. (But it was difficult.)