Modal verbs
MUST & HAVE TO
We use must and have to say that it is necessary to do something. Sometimes it doesn't matter which you use:
Oh, it's later than I thought. I must go. / I have to go.
But there is a difference between must and have to and sometimes this is important:
Must is personal. We use must when we give our personal feelings.
"You must do something" = "I (the speaker) say it is necessary".
She's a really nice person. You must meet her. (= I say this is necessary)
I haven't phoned Ann for ages. I must phone her tonight.
Have to is impersonal. We use have to for facts, not for our personal feelings.
"You have to do something" because of a rule or the situation.
You can't turn right here. You have to turn left. (because of the traffic system)
I have to wear glasses for reading. My eyesight isn't very good.
George can't come out with us this evening. He has to work.
Compare:
I must get up early tomorrow. There are a lot of things I want to do.
I have to get up early tomorrow. I'm going away and my train leaves at 7:00.
Must and have got to
When we say that it is necessary to do something, we use must or have got to:
To get a cheap ticket, you must / have got to book in advance.
Every animal on the island must / has got to be destroyed.
When we want to say that it will be necessary for someone to do something in the future, we use must, have got to, or will have to:
To get there on time, I must / have got to / will have to leave home by 8:30.
Have got to is less formal than the others, and is particularly common in spoken English. We can often use need (to) with a similar meaning:
Before you buy a house, you need to / must / have got to consider all the costs.
Using have got to suggests that someone else or some outside circumstances or authority makes something necessary. We usemust when the speaker decides it is necessary. Compare:
I have to see the head teacher. (…she has called me to her office)
I must see the head teacher. (…I want to discuss something with her)
We prefer have (got) to when we talk about a necessity that is characteristic of a person:
Mary has got to have at least eight hours' sleep a night.
She has to drink two cups of coffee in the morning before she feels really awake.
MUSTN'T & DON'T HAVE TO
Mustn't and don't have to are completely different:
You mustn't do something = it is necessary that you do not do it (so, don't do it)
You must keep it a secret. You mustn't tell anyone. (= don't tell anyone)
I promised I would be on time. I mustn't be late. (= I must be on time)
You don't have to do something = you don't need to do it (you can if you want)
You can tell me if you want but you don't have to tell me. (= you don't need to)
I'm not working tomorrow, so I don't have to get up early.
Mustn't and needn't / don't have to
We use mustn't to say that something is NOT ALLOWED and needn't (or don't need to) or don't have to to say that something is NOT NECESSARY:
You mustn't walk on the grass here.
You mustn't put anything on the shelves until the glue has set hard.
They proved that watching a chess match needn't be boring.
We needn't go into details now, but we seem to agree on the general principles.
SHOULD & OUGHT TO
We can often use should or ought to with little difference in meaning when we talk about OBLIGATION AND PROBABILITY.
Obligation:
- giving advice or making a recommendation:
This soup is cold! You should / ought to send it back.
You'll catch cold if you go out like that. I think you should / ought to take a coat.
- saying what an outside authority recommends (although we prefer should in this case):
The manual says that the computer should be disconnected (passive) from the mains before the cover is removed. (rather than…ought to be disconnected…)
However, we use should (or would), not ought to, when we give advice with I:
I should leave early tomorrow, if I were you. (or I would leave…; or I'd leave…)
- talking about a RESPONSIBILITY OR DUTY:
People should / ought to be warned (passive) of the danger of swimming off this beach.
I should / ought to visit my parents more often.
Probability:
- saying that something is PROBABLY TRUE now or will probably be true in the future:
Have we got any string? There should / ought to be some in the kitchen drawer. (because that's where we always keep it)
You should / ought to have received the report by now.
I enjoyed her first novel, so the new one should / ought to be good.
We can use had better instead of should / ought to, especially in spoken English, to say that we think it would be sensible or advisable to do something. However, we don't use it to talk about the past or to make general comments:
If you're not well, you should / ought to ask Mary to go instead. (or …you'd better ask…) You should / ought to have caught a later train. (not You had better have…)
I don't think parents should / ought to give their children candy. (not …had better…)
When we make a logical conclusion from some situation or activity, we use must not should or ought to:
You must be mad if you think I'm going to lend you any more money.
It's the third time she's been skating this week – she must really enjoy it.
We can use (be) supposed to instead of should / ought to to talk about an obligation to do something. It is commonly used in spoken English to express a less strong obligation:
I'm supposed to be there at 10:00.
The work was supposed to start last week.
We use (be) supposed to when we report what many people think is true:
Eating candy is supposed to be bad for your teeth. (not…should be bad for…)