4. He isn't understand why his brother is angry.

Ответ (EN):

1. The mistake is in the incorrect use of the verb "understand". After the auxiliary verb "isn't", the main verb should be in the base form. The correct sentence is: "He doesn't understand why his brother is angry."

Перевод (RU):

1. Ошибка заключается в неправильном использовании глагола «understand». После вспомогательного глагола «isn't» основной глагол должен быть в базовой форме. Правильное предложение: «He doesn't understand why his brother is angry.»

Ответ: He doesn't understand why his brother is angry.