A/V + 줄 알았다 can used to show that your expectations were defied – ‘I expected/thought X, but’:
한국친구가 모든 외국인이 햄버거를 먹을 줄 알았어요.
My Korean friend thought that all foreigners eat hamburgers (but they don’t).
저는 모든 한국인이 K-팝을 듣는 줄 알았어요.
I thought all Koreans listened to K-Pop (but they don’t).
긴 영화 있는 줄 알았어요.
I thought it would be a long movie (but it wasn’t).
The outcome can be made explicit with 줄 알았는데:
오늘은 비가 올 줄 알았는데 맑은 하늘이에요.
I expected it to rain today but the skies are clear.
톰 크루즈가 키가 큰 줄 알았는데 정말 작아요.
I thought Tom Cruise was tall but he’s really short.
서울에 빨리 갈 줄 알았는데 3시간 이상 걸렸어요.
I thought going to Seoul would be fast but it took more than 3 hours.