The same root does not lose its wāw in every derivative. وَعَدَ retains wāw in the perfect because Rule 1 applies to a particular imperfect configuration.
وَاعِدٌ — one who promises; the active participle keeps wāw.
مَوْعُودٌ — one to whom a promise is made; the passive participle also keeps wāw.
The disappearance of wāw in يَعِدُ does not license deleting it from every derivative.
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Why Wāw Does Not Drop Everywhere
Weak-Initial Roots: Elision, Substitution, Assimilation and Short Paradigms
The same root does not lose its wāw in every derivative. وَعَدَ retains wāw in the perfect because Rule 1 applies to a particular imperfect configuration.
وَاعِدٌ — one who promises; the active participle keeps wāw.
مَوْعُودٌ — one to whom a promise is made; the passive participle also keeps wāw.
The disappearance of wāw in يَعِدُ does not license deleting it from every derivative.