جَاءَ زَيْدٌ — Zayd came: Zayd is nominative and fully declinable.
جَاءَ أَحْمَدُ — Aḥmad came: Aḥmad is nominative and diptote.
Both are proper-name subjects. The difference is the additional verb-pattern cause found in Aḥmad but not in Zayd.
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Zayd Versus Aḥmad
Diptote Causes — Verb Patterns and Their Conditions
جَاءَ زَيْدٌ — Zayd came: Zayd is nominative and fully declinable.
جَاءَ أَحْمَدُ — Aḥmad came: Aḥmad is nominative and diptote.
Both are proper-name subjects. The difference is the additional verb-pattern cause found in Aḥmad but not in Zayd.