We use must and have to talk about obligation. Have to is generally more common than must. The meaning of must and have to is similar in positive sentences. However, the negaitive forms mustn’t and don’t have to have very different uses.
I know that I must study more if I want to the pass the exam next week.
The teacher told us that we have to study for at least 2 hours every night.
Maria will have to study more than that if she wants to pass.
Last year, she had to hire a tutor to help her pass the exam.
Obligation from Within
In British English, we use must when the obligation is implied by the speaker.
- I must study more if I want to pass the exam.
I want to.
- I must stop eating so much sugar.
I have decided to.
Must doesn’t have a future form. Use will have to to talk about future obligations.
musthave to study more, if she wants to pass the exams next month.
Use must to give orders, instructions or advice for the future.
- The exam starts at 9 o’clock so you must be here by 8:45.
Use must have + past participle to express certainty in the past.
- They must have worked hard to get those exam results.
It seems certain that they worked hard.
Have (got) to
Obligation from Outside/American English
When the obilgation comes from outside and in American English, we use have (got) to.
- We’ve got to study every night.
The teacher told me to.
- I have to cut sugar out of my diet.
The doctor told me to.
To talk about future obligation use will/going to have to.
- She’ll have to study more, if she wants to pass the exams next month.
For future arrangements use have got to.
- I’ve got to help Barry with chemistry tomorrow.
Use had to to talk about past obligation.
- They had to hire a tutor to help them with Latin.
It was necessary to hire a tutor.
Use mustn’t when something is not permitted/not allowed.
- You musn’t use a dictionary during the exam.
You are not allowed to use a dictionary.
Don’t Have to
Use don’t have to when something is optional.
- You don’t have to use a dictionary in the exam.
Dictionaries are permitted. Use one if you want.