Question: مَنْ ضَرَبَ؟ — Who struck?
Answer: زَيْدٌ — Zayd did.
The understood full answer is ضَرَبَ زَيْدٌ.
ضَرَبَ: an omitted past-tense verb, recoverable from the question.
زَيْدٌ: nominative subject of that implied verb.
Do not generalize that Zaydun is always a subject: this analysis belongs to the particular question–answer context.
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Subject Rules: Order, Ellipsis and Passive Promotion
Question: مَنْ ضَرَبَ؟ — Who struck?
Answer: زَيْدٌ — Zayd did.
The understood full answer is ضَرَبَ زَيْدٌ.
ضَرَبَ: an omitted past-tense verb, recoverable from the question.
زَيْدٌ: nominative subject of that implied verb.
Do not generalize that Zaydun is always a subject: this analysis belongs to the particular question–answer context.