Have to and must

 

We use have to (do) and must (do) to say that it necessary to do something:

• Oh, it's later than I thought. I {have to / must} go now
• You {have to / must} have a passport to visit most foreign countries.

There is sometimes a difference between must and have to. With must the speaker is expressing personal feelings, saying what he or she think is necessary:

• I must write to Ann. I haven't written to her for ages. ( = The speaker personally feels that he or she must write to Ann.)
• The government really must do something about unemployment. ( = The speaker personally feels that the government must do something.)

With have to the speaker is not expressing feelings. The speaker is just giving facts. For example:

• Karen's eyes are not very good. She has to wear glasses for reading.
• I can't meet you on Friday. I have to work.

You use must to talk only about the present and future:

• We must go now.
• Must you leave tomorrow?

Have to can be used in all forms. For example:

• I had to go to the hospital. (past)
• I might have to go to the hospital. (base form)
• Have you ever had to go to the hospital? (present perfect)

Note that we use do/does/did with have to in questions and negative sentences:

• What do I have to do to get a driver's license? (not "have I to do")
• Why did you have to go to the hospital? (not "had you to go")
• Tom doesn't have to work on Saturdays. (not "hasn't to work")

Mustn't and don't have to are completely different. "You mustn't do something" means "it is necessary that you do not do it":

• You mustn't tell anyone what I said. ( = Don't tell anyone.)
• I promised I'd be on time. I mustn't be late. ( = I must be on time.)

"You don't have to do something" means "it is not necessary to do it; you don't need to do it":

• I don't have to wear a suit to work, but I usually do.
• She stayed in bed this morning because she didn't have to go to work.

You can use "have got to" instead of "have to." So you can say:

• I've got to work tomorrow. or I have to work tomorrow.