Rule 1: 1. After an affirmative sentence: always accusative
When the sentence is affirmative and the general term is mentioned, the excepted noun is put in the accusative, whatever the case of the general term. The general term may be subject, object or the noun after a preposition; it makes no difference.
حَضَرَ الْمُوَظَّفُونَ إِلَّا مُوَظَّفًا وَاحِدًا.
ḥaḍara l-muwaẓẓafūna illā muwaẓẓafan wāḥidan
The employees attended except one employee.
General term in the nominative; excepted noun accusative.
أُغْلِقَتِ الْمَكَاتِبُ يَوْمَ الْعِيدِ إِلَّا مَكْتَبَ الْبَرِيدِ.
ughliqati l-makātibu yawma l-ʿīdi illā maktaba l-barīdi
The offices were closed on the feast day except the post office.
قَرَأْتُ الصُّحُفَ كُلَّهَا إِلَّا صَحِيفَةً وَاحِدَةً.
qaraʾtu ṣ-ṣuḥufa kullahā illā ṣaḥīfatan wāḥidatan
I read all the newspapers except one.
سَلَّمْتُ عَلَى الْوُزَرَاءِ إِلَّا وَزِيرَ الْمَالِيَّةِ.
sallamtu ʿala l-wuzarāʾi illā wazīra l-māliyyati
I greeted the ministers except the minister of finance.
The general term is genitive after عَلَى, yet the excepted noun is still accusative.