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because if I'm not mistaken, they both me "I used to-"
When korean students learn 'I used to', we translate it into '~하곤 했다'.
'~던' is the expression about ~ing.
For example, 학교를 가던 제시카는 친구를 만났다.
= Jessica met a friend going to school
공부하던 제시카가 점심을 먹으러 왔다.
=Jessica who was studying came to have lunch
So ~하곤 했다 is different from ~던 !