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Usage guidance, also known as a tag wiki excerpt, is a short blurb that describes when and why a tag should be used on this site specifically.

Current approaches to morphology (field of studies in morphemes) conceive of morphemes as rules involving the linguistic context, rather than as isolated pieces of linguistic matter. They acknowledge that:

meaning may be directly linked to suprasegmental phonological units, such as tone or stress.
the meaning of a morpheme with a given form may vary, depending on its immediate environment.

When a morpheme stands by itself, it is considered as a root because it has a meaning of its own and when it depends on another morpheme to express an idea, it is an affix because it has a grammatical function.

Food for thought: Every word comprises one or more morphemes.

Links:

SIL's definition and examples

Wikipedia's definition