A systematic change in the vowel of a word root to express different grammatical relationships, like in 'sing, sang, sung.'
From Greek apo- (away) and phone (sound). Also called ablaut, this is a very ancient feature inherited from Proto-Indo-European languages.
Apophony is like nature's way of showing that language is alive and evolving—though it froze centuries ago for most words! English keeps these 'fossils' in irregular verbs, reminding us of patterns that used to be much more productive.
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