It hangs a remembered, unfinished past on a noun: the relationship where the two of you had a bit of a thing, the studying that had been going along fine, the dream I had five or six years ago. Compare the plain past modifier 은, which simply says the thing happened; this one adds that you are looking back and that the action was ongoing or repeated. Two lines here use the frame 든 간에, meaning no matter whether, which is properly spelled 든.