The various hollow-yāʾ forms do not all undergo the same adjustment.
بَاعَ: a moving yāʾ after fatḥah becomes alif — replacement (ibdāl).
يَبِيعُ: the long ī sound reflects the stem’s vowel structure and the management of the medial weak consonant.
لَمْ يَبِعْ: the long medial vowel disappears under jussive contraction — deletion.
بَائِعٌ: medial yāʾ becomes hamzah within the active-participle pattern — hamzah substitution.
Memorizing the surface shape alone does not explain why a particular rule applies.
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Derivational Reasoning: Transfer, Replacement, Deletion
Complete Paradigms of the Hollow Yāʾ Verb: Bāʿa–Yabīʿu
The various hollow-yāʾ forms do not all undergo the same adjustment.
بَاعَ: a moving yāʾ after fatḥah becomes alif — replacement (ibdāl).
يَبِيعُ: the long ī sound reflects the stem’s vowel structure and the management of the medial weak consonant.
لَمْ يَبِعْ: the long medial vowel disappears under jussive contraction — deletion.
بَائِعٌ: medial yāʾ becomes hamzah within the active-participle pattern — hamzah substitution.
Memorizing the surface shape alone does not explain why a particular rule applies.