مَا سَلَّمْتُ عَلَى الْقَادِمِيْنَ إِلَّا زَيْدًا
I did not greet those who arrived except Zayd. زَيْدًا is mansub as mustathna.
مَا سَلَّمْتُ عَلَى الْقَادِمِيْنَ إِلَّا زَيْدٍ
The second form is also allowed. الْقَادِمِيْنَ is majrur after عَلَى, so زَيْدٍ follows it in the genitive case.
In a complete negative sentence, the following option matches the actual case of the group noun.
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مَا سَلَّمْتُ عَلَى الْقَادِمِيْنَ إِلَّا زَيْدًا
I did not greet those who arrived except Zayd. زَيْدًا is mansub as mustathna.
مَا سَلَّمْتُ عَلَى الْقَادِمِيْنَ إِلَّا زَيْدٍ
The second form is also allowed. الْقَادِمِيْنَ is majrur after عَلَى, so زَيْدٍ follows it in the genitive case.
In a complete negative sentence, the following option matches the actual case of the group noun.