Grammatical morphemes are those bits of linguistic sound which mark the grammatical categories of language, for example tense, Number, Gender, Aspect, each of which has one or more functions (Past, Present, Future are functions of Tense; Singular and Plural are functions of Number).
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For example, present progressive "Mommy running". Plural s´ "two books". Possesive s´ "Daddys´ hat"