Modals: must, mustn't, have to, don't have to, needn't, can, can't

Modal verbs are used to indicate modality: likelihood, ability, permission, and obligation.

We use must and have to if we want to say about people's obligations.

• All students must attend their lectures and seminars in the University.
• You must read this book.
• I have to go.

Must not and cannot express prohibition:

• You must not swim fast.
• You cannot go to this party -- you've got a test tomorrow.

Cannot is also used to speak about inability to do something

• You cannot understand Spanish.

Need not and not have to express lack of obligation:

• You need not drive fast. We have got a plenty of time.
• You do not have to wear a dress today. It is very cold.

Verb can is also used to ask people more politely:

• Can you bring me some oranges?
• Can you give him a pencil, please?