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We express greater or lesser degrees of certainty or necessity in different ways. In this unit, we look at the modals and modal-like expressions we use in making statements of certainty or necessity.
Certainty: must, have to, have got to, may, might, can’t, couldn’t, should, and ought to
These modals express varying degrees of certainty. They are used when the speaker is concluding something based on logic and the facts as understood by the speaker. (Remember that when we want to state a fact we are absolutely100 percentsure of, we don’t use modals.)
(Speaker is 100 percent sure.)
Alice knows what’s wrong.
(Speaker is very sure.)
Alice must know what’s wrong.
Alice has to know what’s wrong.
Alice has got to know what’s wrong.
(Speaker is less sure.)
Alice may know what’s wrong.
Alice might know what’s wrong.
Alice could know what’s wrong.
Varying degrees of certainty are expressed in different ways in affirmative and negative statements and in statements about the future. Refer to the Degree of Certainty grammar charts of this unit for examples of some different ways to express these degrees.
The following describes some interesting features of these modals and modal-like expressions.
1. May and might
Some speakers feel that may shows more optimism than might.
They may be able to save their marriage. (more optimistic)
They might be able to save their marriage. (less optimistic)
With these and many other modals and modal-like expressions, however, emphasis and tone of voice, point of view, context, and other factors may color a modal slightly, changing its intensity.
2. Might have and may have
Might have is occasionally used in questions, but may have never is. In negative questions, might normally contracts with not.
Mightn’t he have gone out with his friends?
3. Can’t, can’t have, couldn’t, and couldn’t have
These are used when a speaker feels that it would be impossible to believe or conclude otherwise.
She can’t possibly love me. She’s so mean to me all the time.
You couldn’t have meant what you just said. You must have been very angry.
Can’t and couldn’t can also express feelings of denial, as when something awful has occurred.
John died? Oh, that can’t be! He was so young!
Be careful! Can’t can be used to express certainty, but can is only used to express ability.
4. Should, ought to, may, might, and could
These can be used to express future probability. Should and ought to express a greater degree of certainty, and may, might, and could express a lesser degree.
I heard there’s no traffic. They should arrive on time. (Speaker thinks this is likely.)
They had some car trouble. But they may arrive on time anyway. (Speaker thinks this is possible.)
It’s been raining. They could arrive on time, but traffic always gets so backed up in bad weather. (Speaker thinks this is possible though not likely.)
5. Could have
In statements about the past, could have can express either a lack of certainty or a missed opportunity. Compare these two sentences.
Alice could have told him how mad she was at him, but I’m not sure what she told him. (uncertain guess about the past)
Alice could have discussed the situation with Bruce. It’s unfortunate that she didn’t. (missed opportunity)
Necessity: must, be to, have to, have got to, had better, should, ought to, could, and might
These modals and modal-like expressions are used to express varying degrees of necessity. See the grammar charts of this unit for examples of these degrees.
The following describes some interesting features of these modals and modal-like expressions.
1. Must, have to, and have got to
These modals and modal-like expressions are used when something is required or prohibited by law or formal rules. Statements such as these are obligating; they do not allow for choice in following them. Although must is more formal than have to and have got to, the three forms are equally obligating in this context.
You must have a blood test to get married.
You have to carry your driver’s license with you whenever you drive.
You’ve got to show proper identification to enter this room.
Students must not smoke in the building.
2. Have to and have got to
These can also be used in conversation to make extremely strong suggestions that the speaker wants to insist on, but they are not actually obligating. Suggestions, however strong, do not have to be followed.
You look awful. You really have to see a doctor.
Be careful! Have to in the negative is used to show a lack of necessity.
He doesn’t have to pay for the first counseling session.
3. Must
Be careful! Must and must not are generally not used to make extremely strong suggestions because their use suggests that the speaker has more authority or power than the listener. This would be impolite in many situations.
4. Need not (needn’t), needn’t have
In formal speech and writing, you will hear or see the form need not or needn’t in place of don’t/doesn’t have to. Its past form is needn’t have past participle.
You needn’t be there before the others arrive.
You needn’t have brought a present.
5. Be to and be supposed to
These are used to express strong expectation.
You were to appear in court in October 1. Why weren’t you there?
Noncustodial parents are supposed to pay child support.
Be to carries more power, intensity, and authority than be supposed to and for this reason would be rude in conversation. Be to also carries a threat that there will be consequences if the expectation is not met.
6. Had better
Had better is used to offer strong advice. Since advice, however strong, doesn’t have to be followed, it is not obligating. When a speaker uses had better, though, there is a suggestion of a consequence for not following the advice.
You’d better pay that bill right away.
Be careful! Had better is almost never used in affirmative questions.
7. Had better have
Had better used in the past (had better have past participle) suggests present or past consequences for past actions.
She’d better not have invited her mother! She’s in trouble if she did! (present consequences)
He’d better have gotten there on time! Everyone else did. (past consequences)
8. Should and ought to
These are used to make suggestions and give advice. The speaker wants to convince the listener to do something. Ought to is used less than should in questions, the negative, or the past.
You should go into marital counseling.
You ought to try to communicate better with each other.
9. Might and could
These are used to make suggestions when the speaker is not trying to influence the listener too strongly.
You might try working as a teacher’s aide. You would be good at it. Or you could try writing.
Be careful! Might occurs in the negative, but it doesn’t generally contract with not.
10. Shall
Shall is often used in questions to ask about a course of action. It is only used with I or we.
Shall I open the window?
Yes, please do. It’s really stuffy in here.
When shall is used with we, it is often followed by a sentence with let’s.
A: Shall we go to a movie tonight?
B: No, let’s just stay home and rent a video. I’m too tired to go out.
11. Must have, might have, may have, could have, and ought to have
These combine with had to to make conclusions and guesses about past obligation.
Bruce must have had to go to court.
Ms. Adams must not have had to pay child support.
Alice might have had to leave early.
12. Must, might, may, could, should, and ought to
These combine with be able to to make guesses about future ability.
They may be able to work out their differences.
Modals don’t add -s in the third-person singular form.
She may get married next year. NOT She mays get married next year.
Modals are followed by the base form of a verb without to.
I can get a divorce. NOT I can to get a divorce.
Two modals cannot be used together in the same verb phrase.
We may be able to settle our argument. NOT We may can settle our argument.
Remember that modals and modal-like expressions occur in four patterns.
modal base form:
Bruce and Alice might go into therapy.
modal have past participle:
Alice should have told Bruce she was getting a new dog.
modal be present participle:
Bruce and Alice might be thinking about divorce.
modal have been present participle:
They might have been arguing last night.
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