았던, 였던 thing that I did
았/었/였 is a suffix that conjugates a verb into the past tense. 던, -았/었/였던 becomes a verb ending that makes the verb an adjective that modifies/changes the noun that follows. This form takes the meaning of “something/someone that past tense.” The noun that comes after this verb ending can be either the subject of the verb or the object of the verb, however it is generally very easy to tell during dialog.
Example of 았던:
어렸을 때 살았던 도시 “the city that we lived in when I was little”
지난 주에 먹었던 음식 “The food that we ate last week”
어제 갔던 길에 가라고 “I said go on the road we used yesterday.
었던 여자 좋아 “I like some kind of woman (avoiding the question). (Spoken)