thing vs happening

thing and happening both are nouns.

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

As nouns, happening is a hypernym of thing; that is, happening is a word with a broader meaning than thing:

Other hypernyms of thing include natural event, occurrence, occurrent.

thing (noun)	happening (noun)
a separate and self-contained entity	an event that happens
an action
an artifact
an entity that is not named specifically
any attribute or quality considered as having its own existence
a special abstraction
a special objective
a vaguely specified concern
a statement regarded as an object
an event
a persistent illogical feeling of desire or aversion
a special situation