In linguistics, a morpheme is the smallest meaningful unit of sound within a language. A word is anything consisting of one or more of these units of meaning. A monomorpheme is a word containing exactly one morpheme.
Most monomorphemes are the uninflected form of a word, corresponding typically to the lemma of that word - the version of it which is the "default" form in the speaker's mind.
Talk is a monomorpheme; it cannot be broken down further into smaller components which still have meaning.
Talks and talked are not monomorphemes; both of them contain a second morpheme, /s/ and /ed/, which supply linguistic inflection and modify the meaning away from its default form.
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