previous vs past

previous is not a noun while past is a noun.

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

previous (noun)	past (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)

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too soon or too hasty	earlier than the present time; no longer current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

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