past vs lang syne

past and lang syne both are nouns.

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

As nouns, lang syne is a hyponym of past; that is, lang syne is a word with a more specific, narrower meaning than past:

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

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earlier than the present time; no longer current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

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