You can leave an explanation open, make it sound more likely, report an expectation or express a strong conclusion. The form you choose shows your judgement. It does not guarantee that your judgement is right.
A probability expression tells your reader or listener how you present an idea. You may be considering it, expecting it, or treating it as your strongest explanation. Keep this separate from what is actually true.
| Your intended meaning | Pattern | Example |
|---|---|---|
| A possibility remains open. | may, might, could | The extra charge might cover insurance. |
| You present the possibility as more likely. | may / might / could well | The extra charge may well cover insurance. |
| You expect a normal result. | should | The updated invoice should explain the charge. |
| You draw a strong conclusion. | must | That amount must include the deposit; the balance is much smaller. |
| You strongly reject an explanation. | can't, couldn't | That amount can't include the deposit; the receipt lists it separately. |
Without a probability expression, That amount includes the deposit presents the information as a statement of fact. It is not a certificate of truth: the speaker can still be mistaken. With must, the speaker presents a conclusion reached through reasoning.
More evidence does not automatically select one modal. Different speakers can interpret the same evidence differently. Choose the meaning you want, not a fixed percentage. The strong pair is explained further on Must and can't for deductions.
A core modal takes the base verb without to. It does not take -s with he or she: she might know, not she might knows. You cannot put two core modals together: should be able to, not should can.
Bound and likely are not core modal verbs. The form of be changes with the subject or time: I am likely to, it is likely to, they were likely to. Keep to before the next verb.
For an activity in progress, use modal + be + -ing: The scanner may be checking the next page. To look back at an earlier event, use modal + have + past participle: The scanner may have missed the final page. Full present and past comparisons are on Deductions about the present: must, can't, may, might and could and Deductions about the past: must have, can't have, might have.
Use may, might or could to leave a present or future possibility open. None of these alone tells you that the event will happen.
British Council treats these as alternatives for uncertain possibility and says may is more formal than might or could. Cambridge gives examples with different degrees of certainty but also explicitly notes that speakers disagree about their ranking. Do not learn may = one percentage and might = another.
Cambridge also notes that stress and intonation affect the impression of certainty. The choice of modal is only part of the message. The context and the rest of your sentence matter too.
May can suit a formal notice: Access may be restricted during maintenance. This does not make might or could impossible in formal writing, or may impossible in conversation. Permission and other meanings are covered on May and might: possibility and other meanings and Can, could and be able to: ability, possibility and permission.
Put well after may, might or could when you want the possibility to sound more likely. This adds weight to it without turning it into a proved fact.
Compare The setting might affect the result with The setting might well affect the result. The second gives more weight to the possibility. It still does not say that you have measured the effect.
Its example evidence is not a rule that every may well sentence must describe a surprise. A lesson level, dictionary label and EGP use description are different kinds of evidence.
You can also meet may well not: The second file may well not contain the missing chart. Here you give weight to the negative possibility. Do not confuse well with the different expression may/might as well for choosing a reasonable action; that is covered on May and might: possibility and other meanings.
| Sentence | What you mean in a deduction |
|---|---|
| It may not match. | Perhaps it does not match; the question is open. |
| It couldn't match. | I judge a match impossible or extremely unlikely. |
| It may well not match. | I give more weight to the possibility of no match. |
Could is not simply a past-time word here: couldn't can strongly reject a present explanation. In other contexts it can describe inability or unavailable options. May not can also deny permission. Read the intended meaning from the context rather than assigning every occurrence the same interpretation.
The ordinary negative-deduction counterpart of must is can't or couldn't. Do not use that teaching shortcut to claim that negative must can never express an inference: the advanced attested past pattern is discussed on Deductions about the past: must have, can't have, might have.
Should can say what you expect because it is a normal or reasonable result. It need not be advice. Shouldn't can say that you expect something not to happen.
Compare You should check the cable, which gives advice, with The cable should reach the socket, which reports an expectation. The grammar form alone does not choose the meaning.
British Council teaches the reassuring future use as "if all goes well" and tells you not to use that pattern for unwanted predictions. Follow that guidance when you mean reassurance: The replacement may be expensive states an unwanted possibility without suggesting a normal favourable result.
This is not a universal grammar ban on should next to an unpleasant outcome. Cambridge describes likelihood as a separate use and also records surprise or regret with should. Do not cross out a sentence merely because the event sounds bad. Check its intended meaning and context. Further uses are on Should, ought to and had better: advice and expectation.
Use can for a general tendency or something that is sometimes possible. You are not necessarily making a tentative guess about one event now.
This contrast is about affirmative guesses. It is not a ban on can in every specific situation. Can can express an available option or ability: We can test the connector now; the machine is switched off. That says the test is possible under the present conditions.
Questions can challenge a possibility: Can this reading be correct? Negative can't can reject a specific explanation: This reading can't be correct; the sensor is disconnected. Keep these uses separate from the general-versus-specific contrast.
Could also has a general range of meanings. It can describe an uncertain possibility, a past ability or a conditional option. Do not turn could into a weaker synonym for can in every sentence. See Can, could and be able to: ability, possibility and permission.
Be bound to can present something as certain, inevitable or extremely likely. It describes your prediction, not a guarantee from the world.
Use be + bound + to + base verb, not bound + a bare verb. You can change the tense of be: The arrangement was bound to cause confusion looks back at something you saw as inevitable.
Bound has other meanings. The supplier is legally bound to replace it concerns an obligation, not probability. Context tells you which meaning is intended. Cambridge's dictionary records both meanings, so do not label every bound to sentence as a prediction.
Unlikely and not likely express a low probability. They do not mean impossible. Cambridge describes unlikely as more formal than not likely in its be-expressions reference.
You can modify likely with very or highly and compare probabilities with more or less: The compressed copy is less likely to preserve the fine detail. These do not give an exact numerical probability.
Be likely to does not mean be easy to. The setting is easy to change concerns difficulty. The setting is likely to change concerns probability. Choose the construction that matches your meaning.
To avoid presenting complete certainty, you can use a cautious statement such as I'm not certain that the two versions match. Not certain that does not mean certain that not: a lack of certainty is not a strong negative conclusion.
✗The extra page might contains the key.
✓The extra page might contain the key.
Use the base verb after might.
✗The new layout may to solve it.
✓The new layout may solve it.
May does not take to before the verb.
✗The printer should can handle this size.
✓The printer should be able to handle this size.
Do not combine two core modals.
✗They doesn't might need the second copy.
✓They might not need the second copy.
Put not after might; do not add doesn't.
✗The seam is bound split.
✓The seam is bound to split.
Bound takes to + base verb in this pattern.
✗The two copies is likely to differ.
✓The two copies are likely to differ.
The verb be agrees with the plural subject.
Use Deductions about the present: must, can't, may, might and could for the broad present comparison, Must and can't for deductions for strong conclusions and Deductions about the past: must have, can't have, might have for earlier events. Other meanings are on May and might: possibility and other meanings, Can, could and be able to: ability, possibility and permission and Should, ought to and had better: advice and expectation. Future certainty expressions are on Degrees of certainty about the future. Each linked page keeps its own review status.
Eight questions. Choose one answer for each.