Both must and have to can be used to say that it is necessary to do something.

• You must get up early.
• You have to get up early.

Must, like other modal verbs (may, should, can, etc), is followed by the verb alone (without 'to').
Compare:

• I may come to the party.
• I can tell you tomorrow.
• I must work until 7 p.m.

Common Error: You must to get up early.

Careful:  Must not  does not have the same meaning as  don't have to.

Must not is a polite way to say 'Don't!'

• Don't make any noise! You mustn't wake your father up! (= Don't wake your father up!)

Don't have to means 'It is not necessary.'

• You have to/must get up early during the week.    (= It's necessary.)
• You don't have to get up early on the weekend.    (= It's not necessary.)

Common Error: You mustn't get up early on the weekend. You can get up when you want.

With the meaning 'It was necessary' the past tense of both have to and must is had to.

• You had to get up early yesterday.

Common Error: You must get up early yesterday.

Use have to, not must after 'will,' 'may,' 'might.'

• If it rains, we might/will have to cancel the show.

Must, but not have to, can express the idea: 'I suppose that...'

• Jim's absent! But he's never absent. He must be sick or something.

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