Read about using modal verbs for obligation. Then, when you are ready, try these modal verbs practice exercises.

Obligation and permission with modal verbs

Vampires have to drink blood (must and other modals of obligation).

Use must, have to, don't have to, can and mustn't to talk about obligation and permission.

Vampires must drink blood, not water.

Vampires don't have to kill people - they can just drink a little blood if they prefer.

Vampires mustn't go out in the day or the sun will kill them!

Making positive or negative statements

It's necessary - do it! 

You must do it.

You have to do it.

It's your choice. Do it if you want to.

You can do it.

It's not necessary. Don't do it if you don't want to.

You don't have to do it.

No - don't do it!

You mustn't do it.

You can't do it.

Making questions

Is it necessary?

Do I have to do it?

Is it allowed?

Can I do it?

It is possible to ask questions with "must I do it?", but it sounds more formal and is less common.

Mustn't or don't have to?

The meaning for these is completely different. Mustn't means it is very wrong, or it is a very bad idea. Don't have to means that you don't need to do something (if you don't want to). For example:

You mustn't eat this mushroom - it's poisonous.

You don't have to eat this pizza if you don't like it.

Must or have to?

Often, we use these words to mean the same thing. However, we sometimes use must to show that you think it is necessary, and have to to show that another person thinks it's necessary.

I must do some homework tonight.
(I think it's important to do it)

I have to do some homework tonight.
(my teacher / parents told me to do it)

Present, past and other forms 

In the past, must becomes had to, don't have to becomes didn't have tocan becomes could and mustn't becomes couldn't.

I had to go to the dentist yesterday.

I couldn't wear make-up when I was at school.

If you want to use the present perfect, future or other forms, use have to (for must) or be allowed to (for can) in the correct form.

One day I'll have to learn to drive. (future)

I've never had to work very hard. (present perfect)

I might be allowed to go out tomorrow night. (future possibility)

I love being allowed to go out when I want. (gerund, verb-ing form)

Now test yourself on obligation and permission language with these modal verbs practice exercises.