Grammar
Modal verbs for certainty and possibility
The office is dark, but one desk lamp is on. You might say, 'Someone must be working late.' You are not reporting a rule. You are drawing a strong conclusion from what you can see.
Use modal verbs to make careful deductions about the present and the past, and distinguish a strong conclusion from a possibility or an impossibility.
Read and practice
Read the examples and try the written questions in this guide. Listening recordings are not available for this lesson yet.
Try the written questions
She might have missed the train. /ʃiː maɪt həv mɪst ðə treɪn/
The short answer
Use must when the evidence makes something seem very likely, may, might, or could when it is one possible explanation, and can't or couldn't when the explanation does not fit the evidence. For a past deduction, use modal + have + past participle.
The pavement is wet, so it must have rained overnight.
Why it works: Must have rained is a strong conclusion about an earlier event. The wet pavement is the evidence.
5 more examples
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The bakery lights are on before sunrise, so the staff must be preparing the first batch.
The early lights give a strong reason to think the staff are working.
Not this:The bakery lights are on, so the staff
might be preparingthe first batch. What changes: Might leaves more doubt. The lights could have another explanation. -
This key can't be for the front door because it is far too small.
The size rules out the front door as a reasonable explanation.
Not this:This key
could befor the storage cupboard near the entrance. What changes: Could presents the cupboard as one possible place for the key. -
Ravi has not replied to any messages, so he must have forgotten the appointment.
The lack of replies leads the speaker to a strong conclusion about a past action.
Not this:Ravi
might have forgottenthe appointment, but his phone may have run out of battery. What changes: Might keeps forgetting as one explanation among several. -
The museum is still locked, so the visitors can't have entered through the main gate.
The locked gate makes that past route impossible in this account.
Not this:The visitors
could have enteredthrough the side gate before it was locked. What changes: Could have identifies a possible past route without claiming that it happened. -
The forecast says clear skies, but the match may be delayed if the wind becomes dangerous.
May describes a possible future result that depends on the wind.
Not this:The match
will be delayedbecause the referee has already stopped play. What changes: Will states a firm prediction here because there is direct confirmation.
Read the full guide
Begin with the evidence, then decide how far it takes you. A light under a door may suggest that someone is inside, but it does not prove who. A missing key and an empty coat hook may make one explanation much stronger. English lets you show that difference without claiming more than you know.
Start with evidence, not certainty
A deduction is a conclusion drawn from clues. The clue may be something you see, hear, read, or already know. Before choosing a modal, ask whether the clue makes one explanation very likely, merely possible, or impossible. That question prevents a strong modal from sounding like a guess.
The street is silent and every shop shutter is down. It must be late.
Must be late is a conclusion from the visible clues. It does not express a rule about time.
You see a suitcase by the door and a taxi waiting outside. Is a departure certain, possible, or strongly likely?
It is strongly likely. A natural sentence is, 'They must be leaving soon.' The clues support a strong conclusion.
Use must for a strong present conclusion
Use must + base verb when the evidence strongly supports a conclusion about now. Be is common in this pattern: must be tired, must be at work, or must be the right address. You can also use a continuous form when the action seems to be happening now: must be waiting or must be working.
The baby is asleep, so her parents must be nearby.
The speaker has a good reason to think the parents are near, but does not claim to have seen them.
There are footprints in the fresh snow. Someone must be walking through the park.
The footprints make an ongoing walk a strong explanation.
Choose the stronger conclusion: 'Nila may be at the library' or 'Nila must be at the library,' when her bike is outside and the librarian saw her enter.
Use must be at the library. The bike and the librarian's report give strong evidence.
Keep possibilities open with may, might, and could
Use may, might, or could when an explanation is possible but the evidence does not settle the question. The forms are followed by a base verb: may rain, might arrive, or could belong to the new tenant. They are useful when you want to be accurate about uncertainty instead of making a confident claim.
The parcel might arrive this afternoon, but the tracking page has not updated.
Might tells the listener that an afternoon delivery is possible, not arranged or guaranteed.
The sound could be coming from the ventilation system.
Could names one explanation while allowing other sources for the sound.
A colleague is absent, but you have no message from them. Which modal fits a careful explanation?
Say, 'They may be unwell,' or 'They might be unwell.' Both keep the explanation open.
Use can't or couldn't to rule out a present explanation
Use can't or couldn't + base verb when a suggestion conflicts with the evidence. In this meaning, can't is short for cannot. It does not mean that someone lacks permission. The context tells the listener that you are rejecting an explanation because it does not make sense.
That can't be Dana's coat. Dana left before the rain started.
The timing makes Dana's ownership unlikely or impossible in this situation.
This couldn't be the final version because two pages are missing.
The missing pages rule out the claim that the document is complete.
Why is mustn't wrong in 'That ____ be Leo's phone; he has his phone with him'?
Use can't. Mustn't means forbidden, while can't says the ownership does not fit the evidence.
Look back with modal plus have
To make a deduction about an earlier event, add have and a past participle after the modal. Use must have for a strong conclusion, may, might, or could have for a possibility, and can't or couldn't have for an impossible past explanation. The evidence is often visible now, even though the event happened before now.
The kitchen is clean and the dishes are dry. Amir must have washed them before he left.
The clean kitchen supports a strong conclusion about Amir's earlier action.
The map is missing, so we might have packed it in the wrong bag.
Might have packed gives one possible past explanation for the missing map.
A train left at noon, but you arrived at the station at one. How can you rule out catching that train?
Say, 'I can't have caught the noon train.' Your arrival time makes that earlier event impossible.
Match the modal to the strength of your evidence
A modal is not a substitute for evidence. If you know the fact directly, state it plainly: The manager confirmed the meeting is canceled. If you are inferring, include the clue when it helps: The meeting must be canceled because the room has been reassigned. This habit makes your English clear and fair to the listener.
The screen shows an error message, so the file may not have saved.
The error message supports a possibility, but it does not prove what happened to the file.
The organiser sent a cancellation email, so the workshop will not take place today.
The email gives direct confirmation, so a plain statement is stronger and clearer than a deduction.
When should you choose might instead of must?
Choose might when the clues allow several reasonable explanations. Choose must only when one explanation is strongly supported.
Before you go
Remember this
Use must for a strong deduction, may, might, or could for a possible explanation, and can't or couldn't for an explanation that the evidence rules out. Add have and a past participle when the deduction concerns an earlier event. Keep mustn't for prohibition, not for logical impossibility.
Practice with the written examples
You can practice with the written questions in the Learn tab. Each answer includes an explanation.
Try the written questionsListening test not available yet
This lesson does not have the recordings needed for a listening test. Use the written questions in the guide to check your understanding.
Try the written questionsQuestions about "Modal verbs for certainty and possibility"
What does must mean when I use it for certainty?
For a deduction, must means that the speaker thinks something is very likely because of evidence. The bakery lights example uses must be preparing because the early lights strongly suggest that staff are working. This is different from must in a rule, where it means that an action is required.
The bakery lights are on before sunrise, so the staff must be preparing the first batch.
What is the difference between may, might, and could?
May, might, and could can all present an explanation as possible. In the key example, the side-gate sentence uses could have entered to show one possible past route. Choose one of these forms when the available evidence does not let you make a firm conclusion.
The bakery lights are on, so the staff might be preparing the first batch.
Why do we say can't be instead of mustn't be for an impossible conclusion?
Can't be or couldn't be rejects an explanation because it conflicts with the evidence. The small-key example says it can't be for the front door because the size does not fit. Mustn't be normally means that something is prohibited, so it does not express logical impossibility.
This key can't be for the front door because it is far too small.
How do I make a deduction about the past?
Use a modal followed by have and a past participle. Must have forgotten gives a strong past conclusion, while might have forgotten leaves room for another explanation. The form helps you show both the time of the event and the strength of your conclusion.
This key could be for the storage cupboard near the entrance.
Can I use will instead of a modal of possibility?
Use will when you have a firm prediction or direct confirmation. In the match example, will be delayed fits because the referee has already stopped play. Use may, might, or could when the future result remains possible rather than confirmed.
Ravi has not replied to any messages, so he must have forgotten the appointment.