لِأَنَّهُ يَقْتَرِنُ بِهِ أَوَّلًا
Because it first attaches to the musnad ilayh.
In the sequence of the construction, the separating pronoun comes immediately after the musnad ilayh and before the predicate. In «Zayd huwa al-qāʾim», «huwa» directly follows Zayd.
This is more than physical adjacency: the discussion also establishes semantic identity and formal agreement. Together these explain why the feature belongs in the chapter on the subject.
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The Separating Pronoun (Ḍamīr al-Faṣl): Restriction, Emphasis and Predication
لِأَنَّهُ يَقْتَرِنُ بِهِ أَوَّلًا
Because it first attaches to the musnad ilayh.
In the sequence of the construction, the separating pronoun comes immediately after the musnad ilayh and before the predicate. In «Zayd huwa al-qāʾim», «huwa» directly follows Zayd.
This is more than physical adjacency: the discussion also establishes semantic identity and formal agreement. Together these explain why the feature belongs in the chapter on the subject.