erstwhile vs past

erstwhile is not a noun while past is a noun.

Word	Noun	Adjective	Verb	Adverb
erstwhile	No	Yes	No	Yes
past	Yes	Yes	No	Yes

erstwhile (noun)	past (noun)
	a verb tense that expresses actions or states in the past
	the time that has elapsed
	a earlier period in someone's life (especially one that they have reason to keep secret)

erstwhile (adjective)	past (adjective)
belonging to some prior time	earlier than the present time; no longer current
	of a person who has held and relinquished a position or office

erstwhile (adverb)	past (adverb)
at a previous time	so as to pass a given point