In «مَا زَيْدٌ إِلَّا قَائِمٌ», illā expresses restriction while the predicate stays nominative.
Do not automatically analyze every word after illā as an accusative exception. Here qāʾimun is the predicate, not an accusative mustathnā.
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Restriction and Case Are Different
The Predicate of Mā and Lā Resembling Laysa
In «مَا زَيْدٌ إِلَّا قَائِمٌ», illā expresses restriction while the predicate stays nominative.
Do not automatically analyze every word after illā as an accusative exception. Here qāʾimun is the predicate, not an accusative mustathnā.