Ответы и объяснения

1.We must study hard.
We must listen to the teacher.
2. We mustn't talk to our friends at the lesson.
We mustn't interrupt a teacher.
3.i must help my parents
I must do our homework
4. I mustn't invite my friends, if my parents don't know about it.
I mustn't turn on the music, when my parents are sleeping.