Modal verbs

Obligation Must-Have to
You must pass your exam = You have to be pass your exam
I will have to wear a crash helmet to drive a motorcycle.

No obligation
Helen didn't need to work every day.

Necessity
I need to study very hard to pass my exams.

Prohibition
In spain you mustn't drive on the left.

Advice
You work very hard. you should rest more.
Today is my mother birthday, I had better buy a present.

Ability and knowledge
Mary can speak italian.
She could drive three years ago