Modals of obligation

We use different modal verbs to express obligation in English.

must is used to for written rules or instructions.

The application form must be filled in in black ink.

have to is used to talk about an action that is necessary because of rules or laws, or because someone else tells us to do it

I have to wear glasses because I can't see very well.

must is very similar to have to

It's later than I thought. I must/have to go.

You can use must or have to to give your opinion.

This is a very interesting book. You must/have to read it.

But we use have to to describe an obligation, not our own opinion.

I have to work from 9 till 5.

Although very often we can use must and have to with little difference. We can only use had to when we talk about the past.

We don't have much time. We must/have to hurry.

We didn't have much time. We had to hurry.

We use should/ought to to talk about something that we think is a good idea or a right thing to do.

You look tired. You should go to bed.

We often use should with I think/don't think.

Should is not as strong as must or have to.

You should use this computer. (=I think it's a good idea)

You must/have to use this computer. (= you have no choice)

• We use mustn't when we say that it is necessary that you don't do something, it's against the rules or against the law.

What I'll tell you now is a secret. You mustn't tell anyone.

• We use don't have to/don't need to when we say that something is not necessary.

We don't have/need to hurry. We've got plenty of time left.

• We use shouldn't to say that it isn't a good thing to do.

You shouldn't believe everything you read in the Internet.