فَتَحَ الْوَلَدُ الْبَابَ — The boy opened the door.
فُتِحَ الْبَابُ — The door was opened.
In the active clause الْوَلَدُ is the subject and الْبَابَ the object. In the passive clause فُتِحَ is a passive past verb and الْبَابُ is the nominative passive subject.
This is an explanatory comparison; the book introduces this second type of nominative here, with details to follow.
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Subject Rules: Order, Ellipsis and Passive Promotion
فَتَحَ الْوَلَدُ الْبَابَ — The boy opened the door.
فُتِحَ الْبَابُ — The door was opened.
In the active clause الْوَلَدُ is the subject and الْبَابَ the object. In the passive clause فُتِحَ is a passive past verb and الْبَابُ is the nominative passive subject.
This is an explanatory comparison; the book introduces this second type of nominative here, with details to follow.