chuyển các câu sau sang câu bị động ở thì TL:
They shouldn't sell food on the sidewalks
=> food shouldn't be sold on the sidewalks
we must keep our school clean and tidy
=> our school must be kept clean and tidy
you have to put your pen in the box 
=> your pen has to put in the box
people are going to plant a lot of trees in the yard
=> a lot of trees are going to be planted in the yard