You MUSTN'T / DON'T HAVE TO smoke on the bus. It's prohibited.
MUSTN'T
You SHOULD/ HAVE TO show your passport at the airport control.
HAVE TO
At my school, all children MUST / SHOULD wear a uniform.
MUST
You DON'T HAVE TO / MUST bring the books if you don't want to. It's optional.
DON'T HAVE TO
You SHOULD/ MUSTN'T eat fruits and vegetables. That's my recommendation
SHOULD
You HAVE TO / SHOULDN'T drink cold water if you have the flu.
SHOULDN'T
In the car, you HAVE TO / SHOULD put the seatbelt on, it's the law.
HAVE TO