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When do we use 'must', when do we use 'have to'?

Must and have to both express obligation.
However, they are used differently depending on who imposes the obligation.

The speaker thinks it is necessary, or it is the rule. The obligation comes from an external source.
♦ I must buy flowers for my mother.
(It's her birthday and I decide to do that.)
♦ I have to buy flowers for my mother-in-law.
(It is not my decision - my husband asked me to buy them.)
♦ "You must take more exercise" says the doctor. "It's necessary."
(The doctor thinks it is necessary.)
♦ I have to take more exercise.
(The doctor told me it was necessary.)
♦ I must ask my secretary to book a flight for me.
(It is important for me not to forget.)
♦ I have to call the travel agency.
(My boss asked me to book a flight.)
♦ "Dogs must be kept on a lead."
(Written on a sign in the park which must be obeyed.)
♦ I have to keep my dog on a lead.
(That's what the sign tells me to do.)
♦ "All pupils must wear a uniform."
(The headmaster says it is the rule.)
♦ Do you have to wear a uniform in your school? (Is it compulsory?)
♦ "All passengers must wear seat belts."
(It's the law.)
♦ Do I have to fasten my seat belt? (Is it an obligation?)

'Must' can also be used to show that we are certain something is true. In this case we are making a logical deduction based on the evidence we have.

◊ Julie has had a long journey.  She must  be tired.
◊ There's no heat in this room.  You must  be cold.
◊ Pablo hasn't eaten all day.  He must  be hungry.
◊ I didn't order these books.  There must  be a mistake.

'Must' only exists in the present tense. 'Have to' is used for other tenses.

"I had to go to see the dentist yeaterday."  NOT: "I have must / I musted ..."
"I will have to get up early tomorrow."  NOT "I will must ...."

N.B. In the negative form, the meaning changes.

  • You mustn't tell George =
    it is important not to tell George =
    don't tell George.
  • You don't have to tell George =
    you can tell George if you like,
    but it isn't necessary. It's your decision.
  • Students mustn't talk during the exam.
    (It is forbidden.)
  • You don't have to wear a tie (but you can wear a tie if you like.)

 Try an exercise 

See also:  should - ought to - need

 back to grammar