for certain vs surely

for certain and surely both are adverbs.

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

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

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

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

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