part of speech: auxiliary verb

definition 1: to be forced to; have to or need to.

  • We must breathe in order to live.

definition 2: to be required to; ought to; should;.

  • Children must attend school.

definition 3: used to command or suggest.

  • You must not pull the cat’s tail!

definition 4: used to express, with certainty, some opinion or thought that is based just on what seems logical.

  • The road is wet, so it must have rained.
  • You must be Ellen’s sister! You look just like her!

must or have to?

Have to and must share the meaning of necessity to do something. Have to is used more often in everyday conversation. Must is more often used in writing or in a formal situation. But for the meaning of must in a sentence such as You must be joking!, both have to and must are very common in conversation. You need to be careful, though, to choose between have to and must when your sentence uses not. In negative sentences, have to and must have different meanings, as you can see in the examples below.
Compare:
You have to leave tomorrow.
You must leave tomorrow.
You do not have to leave tomorrow. (It is not necessary for you to leave tomorrow. You can leave tomorrow if you want, or you can stay.)
You must not leave tomorrow. (You do not have permission to leave tomorrow.)

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