If the group is not mentioned and the statement is non-affirmative (negative or interrogative), the clause is incomplete before إِلَّا. In this use, إِلَّا does not itself determine the following noun's case.
The noun after إِلَّا receives the case required by its own role in the sentence: subject, object, or a genitive relationship.
The key question is therefore not 'what follows إِلَّا?' but 'what grammatical job does this noun perform in the sentence?'
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If the group is not mentioned and the statement is non-affirmative (negative or interrogative), the clause is incomplete before إِلَّا. In this use, إِلَّا does not itself determine the following noun's case.
The noun after إِلَّا receives the case required by its own role in the sentence: subject, object, or a genitive relationship.
The key question is therefore not 'what follows إِلَّا?' but 'what grammatical job does this noun perform in the sentence?'