함께 is an adverb meaning "together (with), along with, (formal) in company with" 함께하다 is a verb meaning "share (sth with sb), join" 함께한 is composed of 함께하(다) + ㄴ(determiner particle to make a verb into an adjective) 너와 함께한 시간들이 너무도 행복했었어. I was really happy about the times we shared together. 우리는 함께한 모든 시간을 소중히 간직한다. We cherish every moment we have been together. 한 attached to the end of an action noun means "doing an action" composed of 하(다) + ㄴ