Obligation, necessity, prohibition, advice

Obligation and necessity
  • We use have to to talk about all kinds of obligation. It can be used in all tenses
  • We also use must to talk about obligation. The meaning is similar to have to, but must is usually used when the speaker sees something as a personal obligation.
  • We can use need to to talk about things that are necessary. It can be used in all tenses.
No obligation / no necessity
  • We use not have to when is no obligation to do something.
  • We use not need to when is not necessary to do something.
  • We also can use needn't + infinitive without to to say that it is not necessary to do something.
  • We use mustn't when there is an obligation not to do something. It is mandatory that you do not do something.
  • We use should / shouldn't to give advice or to say what we think is the right thing to do.
  • We can also use ought to / oughtn't to to give advice.
  • We can use must and have to to give strong advice when we think it's very important that someone does something.