☞ How is -았/었/였던 different from -ㄴ/은? ☜
-ㄴ/은 (n/eun) is also a past adnominal suffix, which works as a modifier in the past tense. The difference is that while -ㄴ/은 (n/eun) only represents a plain fact, -았/었/였던 (at/eot/yeotdeon) shows a clear distinction between the past and the present actions/states.
1. 어제 읽은 책 (eoje ilgeun chaek) “the book that I read yesterday” (plain fact)
어제 읽었던 책 (eoje ilgeotdeon chaek) “the book that I read yesterday (but not anymore)” (discontinuation)
2. 자주 간 식당 (jaju gan sikdang) “the restaurant that I often went to” (plain fact)
자주 갔던 식당 (jaju gatdeon sikdang) “the restaurant that I used to go to often” (discontinuation)
☞ How is -았/었/였던 different from -던? ☜
You can also express almost the same thing without the -았/었/였 part because both 았/었/였 and 던 express the past. But without the 았/었/였 part, the sentence changes its meaning a little bit. It still expresses discontinuation between the past and the present, but it expresses more continuation of a past state or an action.
1. 비가 온 날 (bi-ga on nal) “the day when it rained” (plain fact)
비가 왔던 날 (bi-ga watdeon nal) “the day when it rained” (It was raining on that day, but not anymore.)
비가 오던 날 (bi-ga odeon nal) “the day when it was raining” (It was raining when X happened.)
2. 친구랑 마신 커피 (chingu-rang masin keopi) “the coffee that I drank with my friend” (plain fact)
친구랑 마셨던 커피 (chingu-rang masyeotdeon keopi) “the coffee that I drank with my friend”
친구랑 마시던 커피 (chingu-rang masideon keopi) “the coffee that I would sometimes drink with my friend” (a repeated action in the past)