Modals of deduction in English: must be, might be, could be and can't be

You look at the evidence in front of you and you say what you think is true. The kitchen light is on, so somebody is home. The car is gone, so he has already left. English does that job with modal verbs, and it is a different job from the one the same words do elsewhere. Must here has nothing whatever to do with obligation beyond the spelling. You must leave now is an order; she must be home is a guess. Same word, unrelated work. Once you accept that, the rest of the topic is a single scale from certain to impossible, and a short list of forms to hang on it.

The modal verbs page covers the whole family and the grammar rules they share. This page takes one of their jobs, guessing about the present, and does it properly.

The certainty scale

Every deduction you make sits somewhere between "I am sure it is true" and "I am sure it is not true". English marks the points on that line with different modals.

Two things to notice straight away. First, the three middle forms mean effectively the same thing, so the real choice is only between three points, not five. Second, the two ends are must and can't, which do not look like a pair. They are one anyway, and that is the whole difficulty of the topic.

Every form on that table uses the bare infinitive after the modal, exactly as the modal rules require: must be, not must to be; might know, not might knows.

must: the strongest guess, and not an order

Must for deduction means you have worked something out and you are sure of the answer. There is always evidence behind it, spoken or unspoken.

• Her coat is gone. She must be out.
• You have been walking since six. You must be tired.
• He has three houses. He must be rich.

Notice that must is not the same as is. If you knew, you would say she is out. Must be says: I did not see it, but nothing else fits. It is a conclusion, not an observation.

The obligation use of must (you must wear a helmet) shares the spelling and nothing else. You can tell them apart without thinking about it, because obligation must points at a person and tells them to do something, while deduction must points at a fact and says it is true. If you can put "I am sure" in front of the sentence and it still makes sense, it is deduction.

can't: the negative, and the mistake everybody makes

Here is the point of the whole page. When you are sure something is not true, you do not use mustn't. You use can't.

• He can't be forty. He looks twenty-five.
• That can't be her handwriting. She writes with her left hand.
• They can't be asleep. I can hear the radio.

Mustn't means "it is forbidden". You mustn't be late is a rule about behaviour. You can't be late (said while looking at a clock that says half past two) is a deduction about a fact. Learners reach for mustn't be because it looks like the tidy negative of must be, and it is simply not what English does. Get this one thing right and you will sound noticeably more fluent than the average B1 speaker.

The same asymmetry runs through the middle of the scale. The negative of might be is might not be, and that is fine: she might not be home says it is possible she is out. But there is no negative can be in this meaning. She can be home does not work as a guess; use might or could.

might, may and could: the middle ground

These three say the same thing: possible, and I am not committing. Choose on feel rather than on rule.

Could has a quiet second life here. It could be worse and that could be anyone are not really guesses about a specific fact; they open up a range. It is the modal you want when you are listing possibilities rather than picking one.

Watch the negative with could. She couldn't be at home means you are sure she is not, which is the can't meaning, not the "possible" meaning. If you mean "it is possible she is not at home", say she might not be at home.

Deductions about what is happening now

To guess about an action in progress, put the modal in front of be plus the -ing form. The modal supplies the certainty, the continuous supplies the now. The present continuous page covers the -ing form itself.

• She must be working late.
• They can't be sleeping, the light is on.
• He might be waiting outside.

The be is not optional. She must working late is wrong; every one of these forms is modal + be + -ing.

For deductions about the past, English uses modal + have + past participle: she must have left, he can't have known, they might have missed the train. That is a big enough topic to have its own page: see past modals.

Summary table

Common mistakes

Ranked by how often they actually turn up in learner writing.

1. mustn't be for the negative deduction. He mustn't be at home should be he can't be at home. Mustn't is prohibition. This is the error to fix first, because it is the one native speakers notice.
2. Dropping be. She must working, they might sleeping. The modal takes the bare infinitive, and with a continuous meaning that infinitive is be: she must be working.
3. Adding to. It must to be true, he might to know. Core modals never take to, in this use or any other.
4. Using must where English wants the present simple. If you can see it with your own eyes, do not deduce it. You must be Michael is right when you are guessing from a photograph; you are Michael is right when he has just told you his name.
5. can be as a guess. She can be at work does not mean "she might be at work"; it means something closer to "she is capable of being at work", which is nonsense. Use might, may or could.

Practice

Choose the best modal. Answers are below.

1. The office is dark, so it ___ (must / can't) be empty.
2. That ___ (can't / might) be John, he is in Spain this week.
3. I am not sure where my keys are. They ___ (must / could) be in the car.
4. She has read that book four times, so she ___ (must / might) like it.
5. They ___ (mustn't / can't) be out, I can hear the television.

Answers: 1. must (the dark windows are the evidence) 2. can't (you are sure, because he is abroad) 3. could, or might (you are guessing between options) 4. must (four readings is strong evidence) 5. can't (never mustn't for a negative deduction).