-던데요 — Surprise

This expression is the combined form of -더-, which indicates recollection, and -(으)ㄴ데요 which indicates background information and a contradictory or surprising situation.

Accordingly, it is used when expressing something contradictory to what the other person has just said or when expressing the speaker's feelings of surprise toward a past event or particular situation.

Details

1. This expression is also used mid-sentence, but in such cases it is used to indicate a past situation or the fact that the present situation ts contrary to what the case was in the past.

마크 씨는 좋은 사람 같아 보이던데 한번 만나 보세요.

Mark seemed like a good person, so you should meet him.

그 옷이 자야 씨에게 어울리던데 왜 안 샀어요?

That outfit looked good on you, Jaya, so why didn't you buy it?

어제는 많이 출던데 오늘은 따뜻하네요.

Yesterday, it was really cold, but It is warm today.

2. When referring to things that ended in the past, -았/었던데요 is used.

가: 은혜 씨가 잘 지내고 있지요?

Is Eunhye faring well?

나: 네, 얼마 전에 회사를 옮겼던데요.

Yes, I heard she recently changed jobs.

The differences between -더라고요 and -던데요 both of which express recollection, are as follows.

Kimchi Reader

Used to express surprise or disbelief toward a past event or particular situation based on something that the speaker saw or experienced. Expecting an answer from the listener. Can also be used to make a contradiction.