In substitution, hamzah is replaced by alif, wāw or yāʾ. For example, ذِئْبٌ → ذِيبٌ introduces yāʾ in place of hamzah.
In vowel transfer plus elision, the vowel of hamzah passes to the preceding quiescent letter and hamzah then disappears: يَسْأَلُ → يَسَلُ.
In the first case the preceding vowel was already present; in the second, the preceding letter receives a vowel. That difference identifies two distinct morphological causes.
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Vowel Transfer Is Not Substitution
Rules of Hamzated Forms: Lightening, Substitution, Elision and Short Paradigms
In substitution, hamzah is replaced by alif, wāw or yāʾ. For example, ذِئْبٌ → ذِيبٌ introduces yāʾ in place of hamzah.
In vowel transfer plus elision, the vowel of hamzah passes to the preceding quiescent letter and hamzah then disappears: يَسْأَلُ → يَسَلُ.
In the first case the preceding vowel was already present; in the second, the preceding letter receives a vowel. That difference identifies two distinct morphological causes.