The basic case of the fāʿil is nominative. In this example its marker is ḍamma: زَيْدٌ.
Not every fāʿil visibly ends in ḍamma; some nouns use wāw, alif, or another substitute marker. Here زيدٌ is a singular noun, so ḍamma is visible. The essential point is that the syntactic role of fāʿil requires nominative case.
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Al-Marfūʿāt: The Fāʿil, Its Definition and Full Parsing
The basic case of the fāʿil is nominative. In this example its marker is ḍamma: زَيْدٌ.
Not every fāʿil visibly ends in ḍamma; some nouns use wāw, alif, or another substitute marker. Here زيدٌ is a singular noun, so ḍamma is visible. The essential point is that the syntactic role of fāʿil requires nominative case.