Sit down and have a rest.You mustn't sleep in class.
You may read some books.
You moustn't be late for school.
You'd better not litter here.这些都不是“祈使句”
祈使句是表情求，命令的， 一般以动词原形开头！ 你不是说要改为祈使句吗？
那就只能是：
Sit down and have a rest.请把我补充的问题也变成祈使句，Don't sleep in class.
Read some books.
Don't be late for school.
Don't litter here.还有呢Don't sleep in class.
Read some books.
Don't be late for school.
Don't litter here.
Let me use your rubber please.
Please don't play with my litter dog.
Clean the windows.
Don't put your bike here.
Don't ride a bike in the park.