Al-muqaddimah Al-ajurrumiyyah [on the Science of Arabic Grammar]

The Chapter on the MAF‘ŪL MA‘AHŪ

It is an Ism that is Manṣūb and is mentioned (in the sentence) for the purpose of clarifying the one in connection with whom the action is performed, like when you say:

جاء الأميرُ والجيشَ                    (The Commander came withthe army )

استوى الماءُ والخشبةَ       (The water became level withthe stick/wood )

As for the Khabar of كان and its sisters and the Ism of إنّ and its sisters, they have already been mentioned in (the Chapter on the Marfū‘āt) and likewise the the Tawābi‘, they have already been dealt there.