for sure vs for certain

for sure is an adjective but for certain is not an adjective.

Word	Noun	Adjective	Verb	Adverb
for sure	No	Yes	No	Yes
for certain	No	No	No	Yes

As adverbs, for sure and for certain are synonyms defined as:

for sure and for certain: definitely or positively (`sure' is sometimes used informally for `surely')

Other synonyms of for sure include certainly, sure, sure as shooting, sure enough, surely.

for sure (adjective)	for certain (adjective)
not open to doubt

for sure (adverb)	for certain (adverb)
definitely or positively (`sure' is sometimes used informally for `surely')	definitely or positively (`sure' is sometimes used informally for `surely')