"Surely" is an adverb only. "Sure" is an adjective. If formal speech, use these two words as above. Informally, Americans ignore the rules above and use "sure" as both adverb and adjective while rarely using "surely". ----- The dictionary entry below discusses all the common uses of "sure". Merriam-Webster Dictionary http://www.merriam-webster.com/dictionary/sure Usage Discussion of sure Most commentators consider the adverb sure to be something less than completely standard; surely is usually recommended as a substitute. Our current evidence shows, however, that sure and surely have become differentiated in use. Sure is used in much more informal contexts than surely. It is used as a simple intensive and, because it connotes strong affirmation, it is used when the speaker or writer expects to be agreed with <it's a moot point whether politicians are less venal than in Twain's day. But they're sure as the devil more intrusive — Alan Abelson> <he sure gets them to play — D. S. Looney>. Surely, like sure, is used as a simple intensive but it occurs in more formal contexts than sure. Unlike sure it may be used neutrally—the reader or hearer may or may not agree <it would surely be possible, within a few years, to program a computer to construct a grammar — Noam Chomsky> and it is often used when the writer is trying to persuade <surely a book on the avant-garde cannot be so conventional — Karl Shapiro>.