فَإِذَا أَتَتْ جُمْلَةٌ بَعْدَ جُمْلَةٍ
When one clause follows another.
When a clause follows another, the first question is whether the first clause occupies a syntactic position in a larger structure. A clause may function as the predicate of a subject or fill another syntactic slot, even though its internal words have their own grammatical endings.
In “Zaydun yaktubu,” the entire verbal clause “yaktubu” is the predicate of Zayd and occupies the nominative predicate position. An independent opening sentence typically has no such external case position. This distinction determines whether subsequent coordination can share that position.