This grammar pattern is used with a verb or a noun to used to explain an occasion which one saw or experienced in person in a retrospective way.
● 시험이 생각보다 어렵던데요. (The exam was more difficult than I thought (as I recall).)
● 배우가 연기를 아주 잘하던데요. (The acting skills of the actors were great (as I recall).)
● 유리 씨의 남자친구는 태국 사람이던데요. (Yuri’s boyfriend was from Thailand (as I recall).)