وَاعْلَمْ: أَنَّ إِعْرَابَ الْمُسْتَثْنَى عَلَى أَرْبَعَةِ أَقْسَامٍ:
Translation: Know that the case marking of the exception falls into four categories.
The book distinguishes four broad case patterns:
1. Settings where the exception takes the accusative.
2. Complete non-affirmative clauses, with accusative or substitution.
3. Mufarragh clauses, where the actual governor controls the case.
4. The constructions with ghayr, siwā, sawāʾ and ḥāshā.
Do not apply a single case rule indiscriminately.