certainly vs for sure

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

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

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

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

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

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

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