MUST:
1.Police officers must wear a uniform
2.We must obey the rules
3.You must know how to use a computer to work in a bank
MUSTN'T:
1. We mustn't smoke at school
2.You mustn't talk on lessons
3.You mustn't unse a mobile phone on exam
CAN'T:
1. I can speak English,but I can't speak French
2.We can throw rubbish on the street
3.I can't help you because I'm busy