When you draw a strong conclusion from evidence, use must for something you believe is true and can't for something you believe is impossible or extremely unlikely. These forms show your judgement. They do not turn it into a proved fact.
You notice something and decide what it means. The evidence may make you strongly believe one explanation or strongly reject another. You are reporting your reasoning, not necessarily something you have seen directly.
| What you mean | Use | Example |
|---|---|---|
| I strongly believe this explanation. | must | This must be the spare microphone. It is the only one without a label. |
| I strongly reject this explanation. | can't, cannot | This can't be the spare microphone. That one has a blue cable. |
| I present this as a known fact. | A statement without a deduction modal. | This is the spare microphone. I labelled it myself. |
The difference is how you present the information. You can be mistaken in all three cases. Must does not mean that no other explanation is possible in the real world; it means you feel convinced by the evidence.
If you only mean "perhaps", use may, might or could instead. The loose wire might explain the noise leaves the explanation open. The loose wire must explain the noise gives a much stronger judgement. More on Deductions about the present: must, can't, may, might and could.
The base verb is the form without to, such as be, know, need or contain. Must and can't stay the same with every subject: I must, she must, they must. The next verb has no -s.
Use be after the modal, not am, is or are. Do not add do or does to make a modal negative: cannot be, not doesn't can be. Cannot is normally written as one word; can't is its contracted form.
Do not combine two core modals. For a conclusion about ability, use must be able to, not must can: The team must be able to read the code; they have just translated it.
Must can introduce your best explanation of what you notice. You are not giving an order in this use.
Must have does not always refer to the past. In the last example, have means "possess", and the card is with Leo now. Compare the earlier-event form must have taken it in the past section below.
You can also use must when reacting to someone's news, drawing a conclusion about their experience: You are editing the whole catalogue alone? That must take a lot of patience. The deduction can concern a general situation, not only an activity happening at this exact second.
Evidence does not select a modal mechanically. One person might feel convinced and use must; another might be less sure and use might. Say how certain you intend to sound, rather than treating every tired-looking person or lighted window as proof.
Use can't or cannot to reject a possible explanation strongly. In a deduction, can't does not describe someone's ability and does not give a rule.
Could not or couldn't can also reject an explanation about the present: That couldn't be the old timetable; the new logo is on it. Could has other uses too, so the context matters. It is not always a past-time word.
| Compare | Meaning |
|---|---|
| It can't be the entrance code. | I judge that explanation impossible or extremely unlikely. |
| It might not be the entrance code. | Perhaps it is not; I have not decided. |
| It must be the entrance code. | I strongly believe that it is. |
May not and might not do not mean the same as can't. Do not replace perhaps-not with impossible just because both sentences contain a negative.
Use the base verb for a state and be + -ing for an activity in progress. She must own the studio is about ownership. She must be cleaning the studio is about an activity now. Do not use must owning or must is cleaning.
A modal deduction is not itself a present continuous tense: the activity form after it is be + -ing. The modal stays unchanged.
Use have + past participle to make the deduction about an earlier event. A past participle is the form after have, such as packed, eaten, taken or written.
Have stays have, even with he or she: must have removed, not must has removed. Do not invent a past form musted. Couldn't have is another strong negative past pattern. See Deductions about the past: must have, can't have, might have for passive and longer forms.
| Pattern | Meaning |
|---|---|
| She must have the key. | I strongly believe she possesses it now. |
| She must have taken the key. | I strongly believe she took it earlier. |
When you mean an ordinary requirement in the past, use had to, not must have: The driver had to show identification. That says what was required, not what you infer happened. See Have to and must: obligation.
The basic negative counterpart of must for this deduction pair is can't. Mustn't means "it is forbidden" in the rule You mustn't cover this vent. That is different from This can't be the vent; it is a painted mark.
| Meaning | Example |
|---|---|
| Strong negative conclusion | That can't be the emergency switch. It has no electrical connection. |
| Prohibition | You mustn't press the emergency switch during the test. |
| No obligation | You don't have to press anything; the test starts automatically. |
Learn can't as the negative-deduction form, but do not turn this into an absolute claim that negative must never makes an inference. Exact EGP evidence records advanced must not have been, especially with easy. That past pattern is explained on Deductions about the past: must have, can't have, might have. This page does not cross it out as a grammar error.
To ask about a possible explanation, Could...? or Do you think...? gives you a useful pattern: Could this be a charging socket? or Do you think the label might be underneath? BBC's reference prefers these for possibility questions; this does not mean every question with must is impossible.
Must questions can concern requirements rather than deductions: Must we return the samples today? means "Is it necessary?" The form alone does not tell you the intended meaning.
In a short reply you can keep only the words needed to recover the meaning: Is that the original drawing? - It must be; the signature matches. Compare It might be; I'm not certain. The first reply is more confident.
The subject and modal change places in a direct modal question. Do not add do: Could it be...?, not Does it could be...? Question tags and modal replies are covered further on Question tags: isn't it? don't you? and Short answers, so do I, neither do I, and I think so.
✗The box must contains the missing parts.
✓The box must contain the missing parts.
Use the base verb after must, without -s.
✗The serial number can't to match our records.
✓The serial number can't match our records.
No to follows can't.
✗That must is the test result.
✓That must be the test result.
Use be after a modal, not is.
✗The assistant must be sorts the receipts now.
✓The assistant must be sorting the receipts now.
An activity in progress takes be + -ing.
✗She must has sent the replacement.
✓She must have sent the replacement.
In a perfect deduction, have stays unchanged.
✗The date doesn't can be right.
✓The date cannot be right.
Do not use doesn't with can; use cannot or can't.
Compare uncertain possibilities and expectations on Deductions about the present: must, can't, may, might and could, earlier events on Deductions about the past: must have, can't have, might have, may/might on May and might: possibility and other meanings and obligation/prohibition on Have to and must: obligation. Wider probability patterns are on Advanced probability: may, might, could, should and can. Each page retains its own review status.
Eight questions. Choose one answer for each.