auld langsyne vs past

auld langsyne and past both are nouns.

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

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

Other hypernyms of auld langsyne include past times, yesteryear.

auld langsyne (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)

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	earlier than the present time; no longer current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

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