1. MODAL VERBS (I): Must, have to, don’t have to, mustn’t

2. Have to, don’t have to (+) She has to get up at 7 every day.
You have to drive on the left in the UK.
(-) We don’t have to wear a uniform.
He doesn’t have to work today.
(?) Do we have to study this for the exam?
Does he have to tidy his room?

3. Use have to to talk about rules and obligations, or to say that something is necessary.
I have to study harder if I want to pass the exam.
Muslims have to pray 5 times a day.

4. Must, mustn’t (+)You must do your homework tonight.
She must wash the dishes before she leaves.
(-) You mustn’t smoke in class.
They mustn’t leave their bags here.
(mustn’t = must not)

5. Use must + infinitive to talk about rules and obligations.
Must is a modal verb. The only forms are must and mustn’t.
Must, as ALL MODALS, is ALWAYS followed by infinitive WITHOUT TO!!!

6. Must vs. Have to
Both indicate obligation, but there’s a little difference.
It’s getting late. I must go now  obligation from the speaker.
It’s getting late. My mum told me I have to be at home at 9.  obligation from another person.

7. I must go to bed. I’m exhausted.
OBLIGATION
FROM THE SPEAKER
FROM ANOTHER PERSON
I must go to bed. I’m exhausted.
I must remember to phone her tomorrow – it’s her birthday.
You have to wear a seatbelt in the car.
Do you have to work on Saturdays?
I had to wear a uniform when I was at school.

8. No obligation/necessity: don’t have to.
You don’t have to come to the party if you don’t want to.
You don’t have to pay for the photocopies, they are free.

9. Prohibition: mustn’t
Don’t have to and mustn’t are completely different. Compare
You mustn’t smoke inside a hospital. It’s forbbiden. (It’s the law)
You mustn’t drink and drive.
You don’t have to drive to the party, I’ll pick you up. (You can drive if you want, but it is not necessary).