يَا زَيْدُونَ!
Translation: O Zayds!
يَا: particle.
زَيْدُونَ: sound-masculine-plural vocative built on wāw, in accusative position.
The wāw is the fixed ending for this vocative, not proof that the syntactic position is nominative.
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Parsing a Sound Masculine Plural
Al-Munādā: Vocative Types and Rules
يَا زَيْدُونَ!
Translation: O Zayds!
يَا: particle.
زَيْدُونَ: sound-masculine-plural vocative built on wāw, in accusative position.
The wāw is the fixed ending for this vocative, not proof that the syntactic position is nominative.