happen vs happening

happen is not a noun while happening is a noun.

Word	Noun	Adjective	Verb	Adverb
happen	No	No	Yes	No
happening	Yes	No	No	No

happen (noun)	happening (noun)
	an event that happens

happen (verb)	happening (verb)
come to pass
come into being; become reality
happen, occur, or be the case in the course of events or by chance
come upon, as if by accident; meet with
chance to be or do something, without intention or causation