«إن خلت عن ضمير صاحبها الذي تقع هي حالا عنه وجب فيها الواو»
If the circumstantial clause lacks a pronoun returning to its bearer, wāw is required in this construction.
Without that pronoun the later clause does not itself name the earlier participant. Wāw therefore signals the intended connection to the event. In جَاءَ زَيْدٌ وَعَمْرٌو يَتَكَلَّمُ, ʿAmr's speaking may be presented as the attendant circumstance of Zayd's arrival; omitting the clause-level link would obscure that analysis.
Opening interactive controls...
Appendix to Faṣl and Waṣl: Circumstantial Clauses, Wāw and Pronominal Linkage
«إن خلت عن ضمير صاحبها الذي تقع هي حالا عنه وجب فيها الواو»
If the circumstantial clause lacks a pronoun returning to its bearer, wāw is required in this construction.
Without that pronoun the later clause does not itself name the earlier participant. Wāw therefore signals the intended connection to the event. In جَاءَ زَيْدٌ وَعَمْرٌو يَتَكَلَّمُ, ʿAmr's speaking may be presented as the attendant circumstance of Zayd's arrival; omitting the clause-level link would obscure that analysis.