يَا رَجُلُ! — a particular man is addressed: built on ḍamma, accusative position.
يَا رَجُلًا! — an unspecified man is called: overtly accusative.
The distinction is grounded in whom the speaker intends to call.
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The Crucial “Rajul” Contrast
Al-Munādā: Vocative Types and Rules
يَا رَجُلُ! — a particular man is addressed: built on ḍamma, accusative position.
يَا رَجُلًا! — an unspecified man is called: overtly accusative.
The distinction is grounded in whom the speaker intends to call.