أَقَامَ زَيْدٌ؟ — نَعَمْ.
Translation: Did Zayd stand? — Yes.
نَعَمْ: an indeclinable response particle; it is neither a verb nor a subject.
The understood proposition is “قَامَ زَيْدٌ.”
A common mistake is to label naʿam as the subject. It simply affirms the earlier clause; the verb and subject are understood from the question.