From Wiktionary, the free dictionary
Equivalent to 더 ( -deo- , retrospective suffix ) + ㄴ ( -n , past/realis adnominal suffix ) , but it retains the older imperfective sense of 더 ( -deo- ) , not its modern and more nuanced "retrospective " sense.
던 • (-deon )
The imperfective adnominal marker, used to make verbs and adjectives attribute nouns: After verb stems, indicates an action or state which was interrupted and hence not completed. 내 가 어제 먹 던 케이크 nae-ga eoje meokdeon keikeu the cake I was eating yesterday [but I did not eat all of it] After verb stems, indicates an event in the past which happened on a regular basis, but does not continue now. 비 가 오면 나가 걷곤 하 던 대나무 숲 길 bi-ga omyeon naga geotgon hadeon daenamu sup-gil the lane in the bamboo grove I used to walk along when it rained After adjective stems, indicates a state which no longer exists. 아름다웠 던 내 고향 areumdawotdeon nae gohyang my hometown which was once beautiful 
