If the group is mentioned and the statement before إِلَّا is complete and affirmative, the mustathna after إِلَّا must be mansub.
- Complete (تام): the group being discussed is stated.
- Affirmative (موجب): the clause is neither negated nor interrogative.
جَاءَ الْقَوْمُ إِلَّا زَيْدًا
الْقَوْمُ is present and the coming is affirmed, so زَيْدًا is necessarily mansub.