You ( mustn't ) step on the grass.
You ( must) put the books back after you have read them.
You ( mustn't ) talk to the driver when the school bus is moving.
You ( must) clean the classroom every day.
You ( must) wait in line for the school bus.
You ( mustn't) talk too loudly in the reading room.
You (mustn't ) draw on the blackboard.
You ( must) return the books in time.
must的意思是一定要,而mustn't的意思不是不一定要而是不可以