It is used to describe something of the past, but we are not sure if it is still the case now. For example, 예쁘던 대학교 친구 A university friend who was pretty when you met her during your university days, but whether she is still pretty now or not pretty anymore, we don't know. However, if past tense is used, 예뻤던 친구 A university friend who used to be pretty. In this case, it becomes a fact that she was pretty then, but not anymore for now. Same for actions. (1) 먹던 빵 (2) 먹었던 빵 For (1), you were eating a bun, but only finished a portion of it (there is still a portion of it left), whether you are still eating it now, we don't know. For (2), you were eating a bun, and have finished the whole of it. (so the action of eating the bun had ended.)