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English Grammar and Exercises for ESL learners.


Must  -  Have to

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.
MUST HAVE TO
The speaker thinks it is necessary. Someone else thinks it is necessary.
♦ 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.)


'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.

◊ You've had a long journey. You must be tired.
◊ There's no heat in this room.  You must be cold.
◊ You haven't eaten all day. You must be hungry.
◊ I didn't order these books. There must be a mistake.


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.



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