Have to, must and should for obligation and advice
We use have to / must / should + infinitive to talk about obligation, things that are necessary to do, or to give advice about things that are a good idea to do.
Must and have to are both used for obligation and are often quite similar. They are both followed by the infinitive.
I must go now. / I have to go now.
Are these exactly the same?
Well, almost. We often use must for more personal opinions about what it is necessary to do, and have to for what somebody in authority has said it is necessary to do.
I must remember to get a present for Daisy. (my opinion)
You have to look after their hair regularly. (dog experts say so)
Do you have to wear a tie for school? (asking about school rules)
Which verb do people use more?
Have to is more frequent in conversation; must is used more in formal writing, for example in written notices.
Passengers must fasten their seat-belts.
Do they change in form for I, you, he, she, etc.?
Have changes in the third person singular (he/she/it has); but must doesn’t change. It’s a modal verb and modals don’t change.
I think I’ve heard have got to. Is that correct?
Yes, we use both have got to, for obligation, and had better, for advice, a lot in speaking.
You've got to be careful with a cat
You'd better get something a bit quieter.
I'd better go – I mustn't miss the helicopter!
So they’re not used in formal writing?
No. There’s something very important about must and have to. The positive forms are very similar in meaning, but the negative forms are completely different.
You mustn’t forget ...
(don’t forget - you have no choice)
If you don't like him, you don't have to see him again.
(there is no obligation to see him again, but you have a choice)
Umm, I’m still a bit confused ...
Here's an example you can remember:
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.
Ah! Right, I mustn’t forget that.
No, you mustn’t! OK, let’s look at advice, telling people what you think is a good idea. We use should for advice, or making suggestions, and must for strong advice.
You must go for a walk with the dog at least once a day.
Maybe you should go for a coffee or lunch and see how you feel?
You shouldn't leave it on the street.
I think I’ve heard people use should in other ways, like 'he should be here in a minute' – that’s not advice, is it?
No, that’s talking about what is likely or probable. We’ll look at that use another day. We use modal verbs in different ways.
I see. So I should think about one use at a time.
Yes, exactly. You mustn’t get confused by too many uses at once.