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Modal Verbs of Obligation



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!

It's necessary

You must do it.

You have to do it.

It's your choiceYou can do it.
It's not necessaryYou don't have to do it.
Don't do it!You mustn't do it.

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 - it's poisonous.

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

Must or Have to?

Often we use these words to mean the same thing. However, you can 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)

If you want to use any other tense, we always use have to.

One day I 'll have to learn to drive.

I 've never had to work very hard.

Practice these modal verbs