Examples sentences :

• I have to get up at 7am every day. (To get to work on time)
• You have to drive on the left in the UK. (It's the law)
• You have to wear a seatbelt in a car. (It's the law)
• I have to go to a meeting tomorrow. (My boss says so)
• I had to wear a uniform when I went to school. (It was the rule)

Form : ( Don't have to + infinitive without to)

In English we use don't have to + infinitive to say something is not necessary or obligatory. It means you can if you want to, but it's not necessary.

Example sentences :

• He doesn't have to work on Sundays.
• We don't have to wear a uniform at school.
• You don't have to go swimming if you don't want to.
• You don't have to pay for parking here. It's free. 
  

To make questions we use do/does.

Example questions :

• Do I have to go?
• Does he have to bring his CV?

Form : ( Must + infinitive without to )

In English we use must + infinitive to talk about rules and obligations. We normally use must when the speaker imposes the obligation, such as a teacher to students, or even to yourself.

• You must turn off your mobile before boarding the plane. (because you are imposing the rule)
• At this company you must work 40 hours a week. (because you are imposing the rule)
• You must finish the assignment by Friday. (because you are imposing the rule)
• I must lose some weight. (because you believe so)
• I must buy a new shirt for my interview. (It's my own decision)
• You must be on time for the test tomorrow. (Particular occasion)
• I must go to the dentist about my toothache. (because you believe so)

Form : ( Mustn't + infinitive without to )

In English we use mustn't + infinitive to say something is prohibited.

Example sentences :

• You mustn't smoke here.
• You mustn't park here.
• You mustn't drive down this street. It's one-way. (It's against the law)
  

Note : You can often use can't or be + not allowed to instead of mustn't.

Example sentences :

• You can't smoke here.
• You're not allowed to smoke here. (It's prohibited)
• You can't speak loudly in a library.
• You can't park here at rush hours during the week.
  

Note : Mustn't and don't have to have completely different meanings.

Mustn't Vs Don't Have To

• You mustn't go. = You can't go. It's prohibited.
• You don't have to go. = You can go if you want to, but it's not necessary.
  

Note : Have to is a normal verb and exists in all tenses whereas must is a modal verb and its only forms are must and mustn't.

You can also use have to and must for strong recommendations.

Example sentences :

• You must go to Barcelona - it's amazing!
• You have to visit Tate Modern while you're in London. 

Deduction

Form : ( Must / May/ Might / Can't + infinitive without to )

In English we use must when we are sure something is true.

Example sentences :

• He has a big apartment in the city, a holiday house in Spain and he drives a Porsche. He must be rich!
• The lights are on and his car's in the driveway. He must be home. 

In English we use may or might when we think something is possibly true.

Examples sentences :

• He hasn't arrived yet. He might be lost.
• She hasn't called me. She may not have my number.

In English we use can't when we are sure something is impossible.

Examples sentences :

• He can't be ill. I saw him playing tennis this afternoon.
• She can't be English. She has a foreign accent.
• She can't be forty, she only looks thirty!

Note : In this context, the opposite of must is can't, NOT mustn't!

Past Modals

Form : ( Modal Verb + have + past participle (Verb 3) )

In the past, can't, could, may, might and must for probability change to can't have, could have, may have, might have and must have + past participle (Verb 3).

• He can't have seen us waving at him.
• She could have had an accident.
• She may have forgotten to phone him.
• He might have broken a window.
• She must have run to get here on time. 
  




In the past, should and ought to change to should have and ought to have + past participle (Verb 3).

• You should have checked the train timetable before you left.
• She shouldn't have carried so much cash in her bag.
• They ought to have caught the robber by now.




Note : the form of "must" for obligation and "can" are exceptions.

Must for obligation changes to had to + infinitive


• He had to wear a uniform to school.




Can changes to could or was able to


• I could play the piano as a child.
• She was able to escape to safety.