Must, Have to and Should

Must and have to are both used to talk about obligations:

For example:

We must talk to her before she leaves: you think this is very important, and you need to do it. The obligation comes from you and your feelings.

I have to go into work early tomorrow: you have no choice. The obligation comes from the situation, not from you.

Native English speakers use have to much more in conversation. It’s rare to use must to talk about obligations in spoken English, but you can use must to talk about obligations in your written English if you’re writing something more formal.

What about mustn’t and don’t have to?

Must not means that something is forbidden. You are not allowed to do this.

Don’t have to means that something is not necessary. You can choose to do something or not.

In a non-smoking area you mustn’t smoke, but in a smoking area you don’t have to smoke but you can if you want to.

You use should to give advice or express your opinion. For example:

You should go to bed earlier. Then you wouldn’t feel so tired all the time. –> This is my advice. I think it’s better for you to do this, but it isn’t necessary. It’s not an obligation, so you still have a choice.

Another example giving advice:

You should try the fish.

Using must or have to expresses stronger feelings than just using should: it is stronger and more emphatic.

You must try the fish!

You have to try the fish!

However, if you want to give negative advice, only shouldn’t is possible.

You shouldn’t eat so much chocolate at once—you’ll make yourself sick!