يَا عَبْدَ اللهِ: true iḍāfa—ʿabd is muḍāf and Allāh is genitive; no tanwīn on ʿabd.
يَا طَالِعًا جَبَلًا: quasi-construct—ṭāliʿan retains tanwīn and jabalan is an accusative object.
Both vocatives are accusative, but the grammatical tie differs.
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True Construct vs Quasi-Construct
Al-Munādā: Vocative Types and Rules
يَا عَبْدَ اللهِ: true iḍāfa—ʿabd is muḍāf and Allāh is genitive; no tanwīn on ʿabd.
يَا طَالِعًا جَبَلًا: quasi-construct—ṭāliʿan retains tanwīn and jabalan is an accusative object.
Both vocatives are accusative, but the grammatical tie differs.