~던데 (deon-de) – (it) was like that, but
The first meaning of ~던데 is something like ~(은/ㄴ)데 in that it has the meaning of “but”, “although”, “though”, etc.
However, you would use 던데 if you’re thinking in retrospect, usually something that used to be the case but might not be the case now.
In a way, you can translate it to (은/ㄴ)데 but adding “now that I think of it” or “as I recall”.
Example: 지금은 태권도를 잘 하지만 배웠을 때 어때요?
You are really good at Taekwondo now but how was it like when you were learning?
힘이 많이 필요해서 생각보다 더 어렵던데요.
A lot of strength is needed so it was harder than I thought (now that I think of it).