«مَا جَاءَنِي أَحَدٌ إِلَّا زَيْدٌ»
aḥadun: nominative subject.
Zaydun: nominative badal of aḥadun, with ḍammah.
illā: exception marker.
Badal follows the case of its base: because aḥadun is nominative, Zaydun is nominative.
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Negative Clause: Badal Parsing
Al-Mustathnā: Connected and Disconnected Exceptions, Four Case Rules
«مَا جَاءَنِي أَحَدٌ إِلَّا زَيْدٌ»
aḥadun: nominative subject.
Zaydun: nominative badal of aḥadun, with ḍammah.
illā: exception marker.
Badal follows the case of its base: because aḥadun is nominative, Zaydun is nominative.