A functional morpheme (as opposed to a content morpheme) is a morpheme which simply modifies the meaning of the word, rather than supplying the root meaning of the word. That is to say that it functions, but does not mean in and of itself. Examples include inflectional morphemes, like -ing (present tense) and -ed (past tense) and simple affixes like a(n)- (from the Greek, a negation prefix) and -able.