إِنَّهُمْ يَكْتُبُونَ الدُّرُوسَ.
Inna is a governing particle; -hum is its accusative-position noun; yaktubūna is a present indicative verb marked by retention of nūn; the group wāw is the subject in the nominative position; ad-durūsa is the object; the whole verbal sentence is in the nominative position as inna’s predicate.
-hum and the group wāw may refer to the same people but occupy different syntactic functions.