The vocative particle itself is indeclinable. The noun called may be built on a nominative-form sign or may be accusative. In yā Zaydu, the ḍamma does not make Zayd a normal nominal subject.
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Particle versus the Noun’s Case
Vocative Particles: Five Forms, Uses and Parsing
The vocative particle itself is indeclinable. The noun called may be built on a nominative-form sign or may be accusative. In yā Zaydu, the ḍamma does not make Zayd a normal nominal subject.