yesteryear vs past

yesteryear and past both are nouns.

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

As nouns, yesteryear and past are synonyms defined as:

Other synonyms of yesteryear include past times.

yesteryear (noun)	past (noun)
the time that has elapsed	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)

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	earlier than the present time; no longer current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

yesteryear (adverb)	past (adverb)
	so as to pass a given point