certainly vs for certain

certainly and for certain both are adverbs.

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

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

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

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

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