Although many English language learners are often convinced that «must» is stronger than «have to», nowadays both these expressions are used interchangeably to talk about necessity and obligation. Despite the fact that they have similar meaning, there are many other differences in the way the two expressions are used in real, everyday life.
«Must» is a modal verb. This means that «must» does not change according to person (I must, he must) and doesn’t need a modal verb to make a question or a negative form. Example: You mustn't smoke here. Must we talk about this now?
«Have to» is not a modal verb. This means that it changes according to a person (I have to, he has to) and it needs a modal verb to make a question or a negative form. Example: John has to work hard because he has a lot of credit card debt. Do I have to use a pen to fill out this form or can I use a pencil?
Because «must» is a modal verb, it does not have a future or a past form. For future obligations, you can use «will have to». For past, use «had to». Example: Will you have to go to Washington D.C. to get a new passport? I had to wear a uniform when I was in school.
«Have to» is more common in everyday English. People will say things like «I have to get up early tomorrow» or «You have to push this button to start the washing machine» or «Your dog has to be on a leash in public». «Must» is much more formal and you won’t hear it as much.
It’s more common to see «Have to» when asking questions. Example: Do I have to do all these grammar exercises? How often do you have to get up early?
The negative forms of «must» and «have to» have very different meaning:
«Don’t have to» means that something is not necessary. It’s not forbidden and you can do it if you want to. Example: You don’t have to wear a uniform in high school. We don’t have to get Jenna a present because we are paying for her dinner.
«Mustn’t» means something is forbidden. Example: You mustn’t ride a bike without a helmet.
Students often confuse these two forms. Be careful:
Здесь нельзя сидеть.
You don’t have to sit here.
You mustn’t sit here.