lang syne vs past

lang syne and past both are nouns.

Word	Noun	Adjective	Verb	Adverb
lang syne	Yes	No	No	Yes
past	Yes	Yes	No	Yes

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

Other hypernyms of lang syne include past times, yesteryear.

lang syne (noun)	past (noun)
past times remembered with nostalgia	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)

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	earlier than the present time; no longer current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

lang syne (adverb)	past (adverb)
of the distant or comparatively distant past	so as to pass a given point