bygone vs past

bygone and past both are nouns.

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

As nouns, past is a hypernym of bygone; that is, past is a word with a broader meaning than bygone:

Other hypernyms of bygone include past times, yesteryear.

bygone (noun)	past (noun)
past events to be put aside	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)

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well in the past; former	earlier than the present time; no longer current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

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