You must wear a seatbelt. They must

They haven’t had to go to the doctor for years. They’ve very healthy kids.

We can also use ‘have to’ with ‘will’ to describe an obligation, or lack of obligation, in the future. For example::

You’ll have to work hard if you want to get a promotion.

We’ll have to hurry up. The film starts in ten minutes.

She won’t have to start her job until next month, so she’s going on holiday.

Will they have to sign a register during lessons?

I’ll have to train hard for the marathon. I’m not fit enough yet.

You’ll have to tell Marie the truth, otherwise you’ll get into trouble.

Must or Have to?

So when do you use ‘must’ and when do you use ‘have to’? Generally speaking, in the present affirmative form you can use either ‘must’ or ‘have to’. However, there is a slight difference when we speak in the first person. When you want to describe an internal obligation (an order you give yourself), it’s better to use ‘must’. For example:

These pants don’t fit me anymore. I must lose weight.

On the contrary, when you describe an external obligation (an order someone gives you) we use ‘have to’. For example:



My doctor told me I’m overweight and I have to go on a diet.  

Instead when orders are general and official, for example on notices and documents, we usually use ‘must’. For example:

Passengers must keep their bags with them at all times.

In negative sentences you need to remember that if you want to describe an obligation to not do something, you need to use ‘mustn’t’. While if you want to say that there is no obligation, use ‘don’t have to’. For example:

You mustn’t do that! (Don’t do it because it is not permitted.)