كُنْتَ أَنْتَ الرَّقِيبَ عَلَيْهِمْ.
Kunta: defective past kāna with -ta as its nominative noun; anta: in the book’s context a separating pronoun (also analysed as emphasis of -ta); ar-raqība: accusative predicate; ʿalayhim: a prepositional phrase linked to the predicate.
Do not count anta as a new subject while overlooking -ta, which already serves as kāna’s noun.