جَاءَنِي زَيْدٌ وَيَرْكَبُ غُلَامُهُ
Translation: Zayd came to me while his servant was riding.
جَاءَنِي: past verb with object pronoun -ī.
زَيْدٌ: nominative subject.
وَ: circumstantial wāw.
يَرْكَبُ: present verb, indicative with ḍamma.
غُلَامُهُ: nominative subject of yarkabu; -hu links the clause back to Zayd.
يَرْكَبُ غُلَامُهُ: verbal clause in the accusative position as ḥāl.
The ḥāl is now a verbal clause; within that clause ghulām is the subject of yarkabu.