Maybe vs May be

“Maybe” is an adverb meaning “perhaps,” so if you are uncertain whether to use this word or the phrase “may be,” try substituting “perhaps”: “Maybe she forgot I said I’d meet her at six o’clock” becomes “Perhaps she forgot. . . .” When the substitution makes sense, go with one word: “maybe.” When you are wondering whether you may be waiting in the wrong cafe, you’re dealing with a verb and its auxiliary: “may be.” Two words.

Source : http://www.wsu.edu