for certain vs for sure

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

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

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

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

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

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

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