Complex sentence: The class is difficult, and in some ways the brightest are the most disruptive; when he teaches them the complexities of French verbs, they challenge the need to know such things, stating its limited use in modern speech.
Split into simpler sentences: The class is difficult, and in some ways the brightest are the most disruptive. When he teaches them the complexities of French verbs, they challenge the need to know such things, stating its limited use in modern speech.