past vs previous

past is a noun but previous is not a noun.

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

past (noun)	previous (noun)
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)

past (adjective)	previous (adjective)
earlier than the present time; no longer current	too soon or too hasty
of a person who has held and relinquished a position or office	(used especially of persons) of the immediate past
	just preceding something else in time or order

past (adverb)	previous (adverb)
so as to pass a given point