يَا زَيْدُ!
Translation: O Zayd!
يَا: vocative particle.
زَيْدُ: singular proper-name vocative, built on ḍamma, in accusative position.
Visible ḍamma is the fixed ending here. It does not turn the vocative into a nominative subject.
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Parsing the Proper Name
Al-Munādā: Vocative Types and Rules
يَا زَيْدُ!
Translation: O Zayd!
يَا: vocative particle.
زَيْدُ: singular proper-name vocative, built on ḍamma, in accusative position.
Visible ḍamma is the fixed ending here. It does not turn the vocative into a nominative subject.