يَا لَزَيْدٍ!
Translation: O Zayd! (a call for help)
يَا: particle of address/appeal.
لَ: prepositional lām of appeal.
زَيْدٍ: noun genitive after lām; the person appealed to.
Zayd has a kasra because of the lām; it is not an ordinary built-on-ḍamma vocative form.
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Al-Munādā: Vocative Types and Rules
يَا لَزَيْدٍ!
Translation: O Zayd! (a call for help)
يَا: particle of address/appeal.
لَ: prepositional lām of appeal.
زَيْدٍ: noun genitive after lām; the person appealed to.
Zayd has a kasra because of the lām; it is not an ordinary built-on-ḍamma vocative form.