يَدْعُو retains the root-final wāw. With endings, weak consonants and suffixes interact:
يَدْعُوُونَ → يَدْعُونَ — simplification of adjacent weak consonants in the masculine plural.
تَدْعُوِينَ → تَدْعِينَ — adjustment of root wāw with the feminine singular second-person ending.
يُدْعَوُ → يُدْعَى — the passive stem yields the written final alif maqṣūrah.
A single root therefore undergoes multiple distinct operations across its inflectional paradigm.
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Morphological Explanation of the Imperfect
Defective Wāw Verb: Full Paradigm of دَعَا يَدْعُو with Morphological Explanations
يَدْعُو retains the root-final wāw. With endings, weak consonants and suffixes interact:
يَدْعُوُونَ → يَدْعُونَ — simplification of adjacent weak consonants in the masculine plural.
تَدْعُوِينَ → تَدْعِينَ — adjustment of root wāw with the feminine singular second-person ending.
يُدْعَوُ → يُدْعَى — the passive stem yields the written final alif maqṣūrah.
A single root therefore undergoes multiple distinct operations across its inflectional paradigm.