‘Must’ and ‘Have to’ in the positive or question form are used to speak about responsibilities and obligations. Sometimes, ‘must’ and ‘have to’ can be exchanged, but the general rule is that must is used for strong personal obligations (I must do this right now!) and have to is used for responsibilities (I have to file reports every week.) ‘Don’t have to’ and ‘Mustn’t’ have very different meanings. ‘Don’t have to’ is used to express that something is not required. ‘Mustn’t’ is used to express that something is prohibited.
In general, ‘must’ is used in everyday situations in which something important occurs and requires an immediate action. ‘Have to’ is often used to discuss our daily responsibilities and is commonly used to describe workplace situations. Many times, ‘have to’ is used in place of ‘must’ in informal English.
Listed below are explanations, examples and uses of must / have to / mustn’t / not have to
Have to do – Responsibilities
Use ‘have to’ in the past, present and future to express responsibility or necessity. NOTE: ‘have to’ is conjugated as a regular verb and therefore requires an auxiliary verb in the question form or negative.
We have to get up early.
She had to work hard yesterday.
They will have to arrive early.
Does he have to go?
Must do – Obligations
Use ‘must’ to express something that you or a person feels is necessary. This form is used only in the present and future.
I must finish this work before I leave.
Must you work so hard?
John must explain this if he wants his students to succeed.
It’s late. I must get going!
Don’t have to do – Not Required
The negative form of ‘have to’ expresses the idea that something is not required. It is however, possible if so desired.
You don’t have to arrive before 8.
They didn’t have to work so hard.
We don’t have to work overtime on Saturdays.
She didn’t have to attend the presentation.
Mustn’t do – Prohibition
The negative form of ‘must’ expresses the idea that something is prohibited – this form is very different in meaning than the negative of ‘have to’!
She mustn’t use such horrible language.
Tom. You mustn’t play with fire.
You mustn’t drive more than 25 mph in this zone.
The children mustn’t go into the street.
IMPORTANT: The past form of ‘have to’ and ‘must’ is ‘had to’. ‘Must’ does not exist in the past.
Watch the video below so you won’t have any more doubts about this subject!