Réponse:
1.The students must respect their teachers.
(Must indicates obligation or necessity)
2.What can I do for you sir?
(Can indicates possibility or ability)
3.May I open the door please?
(May indicates permission in a polite question)
4.If you want to pass the pas test, you must study your.
(Must indicates obligation again)
5.All citizens must obey the law.
(Must emphasizes the importance of following the law)