A deduction is a conclusion you reach from information, rather than something you have checked directly. Modal verbs let you show a strong belief, an uncertain possibility or an expectation. They do not prove that your conclusion is right.
First decide what you want to say. Are you strongly convinced, considering one possible explanation, or saying what you normally expect? The same information can lead different people to different conclusions.
| Your meaning | Form | What you are saying |
|---|---|---|
| Strong positive conclusion | must | I am convinced this is true, from the information I have. |
| Strong negative conclusion | can't, cannot; also couldn't | I judge this impossible or extremely unlikely. |
| An uncertain possibility | may, might, could | This is one possible explanation; I am not sure. |
| An uncertain negative possibility | may not, might not | Perhaps this is not true. |
| An expected situation | should, shouldn't | This is what I reasonably expect, rather than a guarantee. |
Do not give may, might and could fixed percentages. They overlap. Context, stress and the speaker's view affect how certain they sound. A choice between may and might is not a reliable test of an exact amount of certainty.
The modal does not take -s with he, she or it. The next verb has no to: might contain, not might contains or might to contain. Use be, not am, is or are, after the modal. Do not put two core modals together: may be able to, not may can.
Put not after may or might without don't or doesn't. Mightn't is a contracted form of might not. May not has no ordinary contracted form. Cannot is normally one word; can't is its contraction.
Use must when the information makes you feel that one explanation is right. This is your conclusion, not an instruction and not a statement that you have independently verified it.
A strong conclusion can be wrong. You might later discover that someone printed new envelopes with the old logo. Must reports your confidence at that moment; it does not remove other real-world possibilities.
| Sentence | How you present the information |
|---|---|
| The spare cable is in drawer three. I put it there myself. | You state something as a fact. |
| The spare cable must be in drawer three. It is not in either of the other drawers. | You give a conclusion from your search. |
A statement without a modal presents something as true; grammar alone cannot guarantee that the speaker's information is correct. The difference here is how you present your knowledge.
Use can't or cannot when you judge an explanation impossible or extremely unlikely. Couldn't can also express this judgement about a present situation. It does not automatically put the situation in the past.
Keep the judgement separate from proof. A speaker who says can't may be mistaken about the evidence, just as a speaker who says must may be mistaken.
The basic negative counterpart of deductive must is can't, not an automatic replacement with mustn't. Mustn't is the form you need for a prohibition: Visitors mustn't touch the glass. Do not learn an absolute claim that negative must can never express an inference: the past reference includes attested advanced must not have been patterns. For the present comparison here, use can't or couldn't to state a strong negative conclusion.
Use these forms when an explanation is possible but you are not ready to say it is the answer. You can put several possibilities side by side.
All three can express the same uncertain explanation here. May is more formal than might and could in the British Council comparison, but these are not separate grammatical meanings. Do not mark a possible might answer wrong just because a key prefers may.
May not and might not mean that the negative situation is possible. They do not mean that the positive situation is impossible.
| Example | Meaning |
|---|---|
| The spare screen might not work. | Perhaps it does not work; we have not checked. |
| The spare screen may not work. | The same negative possibility. |
| The spare screen can't work without a power supply. | The speaker judges working impossible in those conditions. |
Could not is not the ordinary negative equivalent of uncertain could. It can express impossibility, inability or lack of permission, depending on context. Read the whole sentence, not just the word not.
Use the base verb for a state, such as know, own or need. Use be + -ing when you mean an activity in progress. You do not need an -ing form just because your conclusion concerns now.
| Pattern | Example | Time you mean |
|---|---|---|
| modal + base verb | He may own the workshop. | A present state. |
| modal + be + -ing | He may be painting the workshop. | An activity in progress. |
| modal + have + past participle | He may have painted the workshop. | A possible earlier action. |
Could be painting can refer to now even though could also has past uses elsewhere. For earlier deductions, use the perfect patterns on Deductions about the past: must have, can't have, might have; don't change must to musted.
Could...? and Do you think...? are useful ways to ask about an explanation. These are more usual than questions with may, might or must for this purpose.
Questions can also challenge a possibility: How can both clocks be right? Can is not banned from every question about possibility. Do not add do to a direct modal question: Could it be...?, not Does it could be...?
In a reply you can leave out words that are already understood: Could the sound be the lift? - It could be, but I haven't checked. A reply such as It must be presents a stronger conclusion than It might be.
Can describes something that sometimes happens or is capable of happening. For one uncertain explanation of what is happening now, use may, might or could instead.
| Example | Meaning |
|---|---|
| Old wooden doors can make loud cracking sounds. | This sometimes happens with such doors. |
| This sound could come from the wooden door. | That is one explanation of this particular sound. |
Can also expresses ability and permission. We can enter through the side door can describe an available option, not a guess about where somebody is. A grammatical can sentence is not wrong merely because it refers to one occasion; check its meaning.
Should and shouldn't can say what is likely or reasonable to expect, rather than give advice. They do not carry the same meaning as must or can't.
This expectation can fail. If you are simply warning of an unwanted possibility, may, might or could gives a clearer meaning: The replacement part might jam. British Council teaches should in this probability use as "if all goes well". This is not a reason to cross out every sentence about an unwelcome result without looking at the context.
Compare an expectation, The lid should close now; the obstruction is gone, with advice, You should close the lid before moving the box. The situation tells you which meaning is intended. More on Should, ought to and had better: advice and expectation.
Put well after may, might or could to make the possibility sound more likely. It does not turn it into a verified fact.
Well belongs between the modal and the following verb. Compare might be with might well be, not a fixed numerical scale of may, might and could. More advanced probability patterns are on Advanced probability: may, might, could, should and can.
✗The password must belongs to another account.
✓The password must belong to another account.
The verb after must is in the base form, without -s.
✗The attachment might to contain a map.
✓The attachment might contain a map.
Do not add to after a core modal.
✗The printer may is offline.
✓The printer may be offline.
Use be after a modal, not is.
✗She doesn't might recognise the symbol.
✓She might not recognise the symbol.
Put not after might; do not add doesn't.
✗Does the missing file could be on the backup drive?
✓Could the missing file be on the backup drive?
Put could before the subject without does.
✗The operator must be checks the numbers.
✓The operator must be checking the numbers.
For an activity in progress, use be + -ing.
Continue with Must and can't for deductions for the strong positive/negative pair and May and might: possibility and other meanings for possibility and other meanings. See Deductions about the past: must have, can't have, might have for earlier events, Advanced probability: may, might, could, should and can for wider probability patterns and Degrees of certainty about the future for future predictions. The linked pages have separate review statuses in the completion ledger.
Eight questions. Choose one answer for each.