وَحُكْمُهُ فِي تَوْحِيدِ فِعْلِهِ وَتَثْنِيَتِهِ وَجَمْعِهِ وَتَذْكِيرِهِ وَتَأْنِيثِهِ عَلَى قِيَاسِ مَا عَرَفْتَ فِي الْفَاعِلِ.
Translation: Its verb follows the same principles of singular, dual, plural, masculine and feminine agreement previously learned for the subject.
The author refers to the earlier agreement rules for the fāʿil. Crucially, an overt dual or plural noun normally follows a singular verb in standard verb-initial order; a pronominal passive subject is reflected in the verb’s person/number form.