Grammar
-던
recalled past action
Form
Verb or adjective stem + -던 + noun — “that used to / was ...-ing”.
Function
Recalls a past action or state, often habitual or viewed as unfinished at the reference point.
Why this form?
-(으)ㄴ — “that did / is”: more readily describes completion or a property without the retrospective viewpoint.
Forms not to confuse
-(으)ㄴ
어제 읽은 책이에요. — It is the book I read yesterday.
Examples
문을 열려고 하던 소리였어요? — Was that the noise you made while trying to open the door?
제가 자주 가던 카페예요. — This is the café I used to visit often.