Why did the word "عدوًا" (enemy) appear with a fatḥah (accusative case) in the verse: "إِنَّ مِنْ أَزْوَاجِكُمْ وَأَوْلَادِكُمْ عَدُوًّا لَّكُمْ فَاحْذَرُوهُمْ" (Indeed, among your wives and your children are enemies to you, so beware of them)?

The word " عدواً" (an enemy) is in the accusative case because it is the noun of "inna" (اسم إنّ). Originally, it was a subject whose predicate was a prepositional phrase that preceded it. "Inna" put it into the accusative case when it entered upon it, and this is the function of "inna" and its sisters.