1: We use the modals must, need to, and have to for actions that are obligations.
- You must hurry.
- You need to finish this.
- I have to go now.
- We need to pay the bill.
2: All the modals are very similar in meaning but must is stronger than have to or need to.
- You must take your medicine. (strong)
- You have to take your medicine. (not as strong)
- You need to take your medicine.
3: The negative forms have very different meanings. Must not or mustn't is a command to not do something.
- You mustn't go there. / You can't go there.
- She mustn't trust him. / She can't trust him.
- They mustn't be late. / You can't be late.
- You mustn't lose this. / You can't lose this.
4: When we say you don't need to, or don't have to do something, then it is an option or not necessary.
- You don't have to wear a tie. / It is OK not to wear a tie.
- You don't need to be there. / It is OK not to go.
- She doesn't have to pay me back. / She can keep her money.
- He doesn't need to ask her. / It is OK to not see her.