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What is the definition of morpheme? The smallest grammatical or meaningful unit?

“In linguistics, a morpheme is the smallest grammatical unit in a language. In other words, it is the smallest meaningful unit of a language.” Yet, this definition sometimes appears to be troublesome because ‘the smallest grammatical unit’ and the smallest meaningful unit’ do not seem to be in a one-to-one relation. Past tense
(1) walk-past = walk-ed
semantic unit [past] = grammatical unit [-ed]

(2) a. burn-past =burned/burnt (similar: Dreamed or dreamt) semantic unit [past] = grammatical unit [ed] / [t] b. Knitted vs. knit Knit-[past] = grammatical unit [ed]/[0] In (1) the smallest semantic unit [past] is the equivalent of the smallest grammatical unit. They are in one-to-one relation. On the other hand in (2) the smallest semantic unit [past] has two grammatical forms. What is the morpheme for the past tense in (2)? Is it [past] or /ed/? If the latter is the answer, why is [past] wrong/not used as a morpheme? Why is the definition of “grammatical unit” used, independently from “semantic unit” even though the semantic unit [past] appears to be the equivalent of the “abstract” grammatical unit /ed/? Why is it the case that the two types of concepts for a morphine needed? One more relating question is regarding allomorphs. “Allomorphs are variants of a morpheme that differ in pronunciation but are semantically identical.” The word ‘morpheme’ appears to mean “abstract grammatical unit” that is uniquely connected with a specific semantic unit. Is this interpretation correct? If so, again why is it the case that the two types of concepts for a morphine needed?

Thanks KF