Use 인줄알았어요 after a noun, with or without a final consonant. 시작시간인줄알았어요 reports what you thought then. Here we use it when later information differs.
Situation or form
Korean expression
시작시간 + 인줄알았어요
세시가시작시간인줄알았어요.
준비시간 + 인줄알았어요
세시가준비시간인줄알았어요.
시작하다 → 시작하는줄알았어요
세시에시작하는줄알았어요.
가다 → 가는줄알았어요
바로가는줄알았어요.
Use 인줄 with a noun and -는줄 with an action. 알았어요 does not make your belief someone else’s promise. You can report it using the previous unit: 준은 …인줄알았다고했어요.