for certain vs certainly

for certain and certainly both are adverbs.

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

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

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

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

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