past vs yesteryear

past and yesteryear both are nouns.

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

As nouns, past and yesteryear are synonyms defined as:

Other synonyms of past include past times.

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

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earlier than the present time; no longer current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

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