An allomorph is a different form of a Morpheme. The regular Simple Past ending is -ed. In the verb 'advised' the ending is pronounced /d/, but in 'walked' it is pronounced /t/ and in 'wanted' it is pronounced /i:d/. A verb ending in -e, like 'hire' only takes -d. These are different forms of the same thing; they are allomorphs of the simple past tense ending.
'Allomorphs' - Related Links
Grammar Topic: Pronunciation
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