An affirmative statement asserts an event without negating or questioning it.
«جَاءَ الْقَوْمُ إِلَّا زَيْدًا» = The group arrived except Zayd.
Complete: the group is named.
Affirmative: no negation is present.
Hence the exception is accusative: Zaydan, not nominative Zaydun by the ordinary substitution option.
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Al-Mustathnā: Connected and Disconnected Exceptions, Four Case Rules
An affirmative statement asserts an event without negating or questioning it.
«جَاءَ الْقَوْمُ إِلَّا زَيْدًا» = The group arrived except Zayd.
Complete: the group is named.
Affirmative: no negation is present.
Hence the exception is accusative: Zaydan, not nominative Zaydun by the ordinary substitution option.