must;
you must tidy up your room
we must save energy
students must arrive at school on time
students must join the activites in class
students must listen it the teacher at school
mustn't;
you mustn't drop litter in the street
have to mustn't talk to each other during the lesson
students mustn't eat in the classroom
students mustn't write on the desk and they mustn't bring their pets to the classroom
