Dialogue

ALEXIS: Hi Mayra

Mayra: Hello Alexis

A:How are you?

M: Fine thanks

A: and what you doing?

M: I'm studying for the English exam

A: Ok and you study for the Mathematics exam?

M:Ohh Nooo!!

A: I think you should study for the Mathematics exam

A: Well see you later because i have to return this book to the library

M: Ok see you later because i must be in the classroom at nine o'clock

A: Bye

Should/Shouldn't

*Peaple shouldn't be allowed to have trash in the streets

*You should go to the doctor

Dont/Have to

*We use have to to express necessity

I have to return the books to the library

*We use don't/doesn't have to to express lack of necessity

You don't have to wear a uniform at work

My exposition

Must/Musn't

*We use must to express a rule, obligation or duty

You must wear seatbelt

*We use musn't to express prohibition

You musn't talk in class