Must, have to
Must/mustn’t
Use must + infinitive without to
Must is a modal verb, and modal verbs are followed by an infinitive without to.
I must go to the doctor.
Modal verbs don’t add -s after he/she/it.
He must come with us.
Use must/mustn’t in negatives and questions
Do not use do/did in negatives and questions.
We mustn’t arrive early.
Must we arrive early?
Have to/don’t have to
Use do and did in negatives and questions
The verb have in have to is not an auxiliary verb. Use do/don’t, does/doesn’t, did/didn’t in questions and negative sentences in present and past.
Do you have to get up early tomorrow?
Did you have to buy a new sound card?
We don’t have to stay until the end.
No contraction
The verb have in have to cannot be contracted to ‘ve.
I have to study for the exam tomorrow.
Past and future forms
Only have to has a past and a future form. Must does not have a past or a future form.
Yesterday, I had to take a taxi.
You will have to take a taxi if it rains tomorrow.
Be careful about the difference between mustn't and don't have to!
Mustn't means it's not allowed, or it's a bad idea:
You mustn't eat so much chocolate, you'll be sick.
Don't have to means you don't need to do something, but it's fine if you want to do it:
I don't have to get up early at the weekend.