past vs auld langsyne

past and auld langsyne both are nouns.

Word	Noun	Adjective	Verb	Adverb
past	Yes	Yes	No	Yes
auld langsyne	Yes	No	No	No

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

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earlier than the present time; no longer current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

past (adverb)	auld langsyne (adverb)
so as to pass a given point