past vs langsyne

past and langsyne both are nouns.

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

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

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earlier than the present time; no longer current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

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