~ㄴ/은: Attached to a verb to describe a noun where the action occurred sometime in the past. There is no additional meaning given to it. All we know is that at some point in the past, the action happened.
~던: Attached to a verb to describe a noun where the action is recalled to have occurred repeatedly in the past, and continually repeating to the present (or to the time described in the sentence).
~았/었던: Attached to a verb to describe a noun where the action is recalled to have occurred in the past, but has finished occurring and currently does not occur.
I don’t like distinguishing these grammatical principles based on their English translations. Given the number of possible situations that could come up, there is no perfect way to accurately translate any of them. However, the examples below show common translations for each of these grammatical principles in use:
내가 입은 바지 = the pants I wore
내가 입던 바지 = the pants I used to wear
내가 입었던 바지 = the pants I had worn.(I din't wear the pants anymore)