In «مَا جَاءَ أَحَدٌ إِلَّا زَيْدٌ», aḥadun is the subject and Zaydun is badal.
In «مَا جَاءَ إِلَّا زَيْدٌ», Zaydun itself is the subject.
The case ending is identical, but the grammatical relation is not.
Opening interactive controls...
Mistake: Calling Every Nominative a Subject
Al-Mustathnā: Connected and Disconnected Exceptions, Four Case Rules
In «مَا جَاءَ أَحَدٌ إِلَّا زَيْدٌ», aḥadun is the subject and Zaydun is badal.
In «مَا جَاءَ إِلَّا زَيْدٌ», Zaydun itself is the subject.
The case ending is identical, but the grammatical relation is not.