for sure vs certainly

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

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

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

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

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

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

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