The book's explanations distinguish several changes in the active perfect:
دَعَوَ → دَعَا — final wāw becomes alif after fatḥah.
دَعَوَا — the dual suffix follows a retained root wāw.
دَعَوُوا → دَعَوْا — the masculine plural contracts a sequence of wāws.
دَعَوَتْ → دَعَتْ — after the weak-letter adjustment, two sukūns lead to deletion of the long vowel.
These are not one undifferentiated change: substitution and deletion have different triggers.
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Morphological Explanation of the Perfect
Defective Wāw Verb: Full Paradigm of دَعَا يَدْعُو with Morphological Explanations
The book's explanations distinguish several changes in the active perfect:
دَعَوَ → دَعَا — final wāw becomes alif after fatḥah.
دَعَوَا — the dual suffix follows a retained root wāw.
دَعَوُوا → دَعَوْا — the masculine plural contracts a sequence of wāws.
دَعَوَتْ → دَعَتْ — after the weak-letter adjustment, two sukūns lead to deletion of the long vowel.
These are not one undifferentiated change: substitution and deletion have different triggers.