past vs erstwhile

past is a noun but erstwhile is not a noun.

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

past (noun)	erstwhile (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)

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earlier than the present time; no longer current	belonging to some prior time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

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