Speculation and deduction: Modal verbs and expressions

Modal verbs of speculation and deduction

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Must

We use must + infinitive for obligation and strong recommendation, but also to say that we are quite sure that something is true.

• They are almost identical. They must be sisters.
• But I answered all the questions correctly. There must be a mistake. 

We use must be + –ing with dynamic verbs to say that we are quite sure that something is happening now.

• What you are saying is not possible. You must be kidding.  
• Can you hear the shouting? Our neighbours must be arguing again. 

We use must have + past participle or must have been + -ing to say that we are quite sure that something was true or happened in the past.

• I can’t find my wallet! I must have dropped it in the taxi.
• You must have had a real scare when you saw the crocodile. 
• He was the only victim. He must have been sleeping when the fire started.

Can’t

We can use can’t + infinitive to say that we are quite sure that something is NOT true.

• He can’t be that famous. I’ve never heard his name before. 
• She can’t be married. I have never seen a ring on her finger. 

We use can’t be + –ing to say that we are quite sure that something is NOT happening now.

• They can’t be travelling by bus. There is no bus service on Sundays. 

We use can’t/couldn’t have + past participle or can’t have been + -ing to say that we are quite sure that something did NOT happen or was NOT true in the past.

• You can’t/couldn’t have seen John last night. He was in hospital. 
• She can’t/couldn’t have passed the test. She didn’t even open the books. 

❗ Note that for negative deduction, we use can’t (NOT mustn’t):

• He mustn’t be that famous.
• He can’t be that famous.

Could/might/may

We can use could/might/may + infinitive to say that it’s possible that something is true (but we are not sure).

• I can’t get hold of him on the phone. He could/might/may be away. 
• We didn’t do anything when we could, and now it could/might/may be too late.

We use could/might/may be + –ing to say that we are quite it’s possible that something is happening now (but we are not sure).

• Tom isn’t at the office. He might/may be working from home today.  
• ‘Why isn’t she picking up her phone.’ ‘She might/may be driving.’

We can use could/might/may have + past participle or might/may have been + -ing to say that it’s possible that something was true or happened in the past.

• It’s been three days. They could/might/may have finished painting the house by now. 
• If they left at 9, they could/might/may have already arrived.
• She was home last night but didn’t answer the door. She might/may have been having a shower.

We can use could have + past participle to say that something was possible in the past but did not happen.

• He could have played in the first division but had a terrible injury.
• He could have had better jobs if he had spoken English.   

❗ Note that we use might not or may not (NOT could not) to talk about a negative possibility.

• He seems too calm. He might/may not be aware of the risks. 
• She might/may not have heard us. Knock again. 

Should/ought to

We use should/ought to + infinitive to talk about a situation that we expect to happen (present or future).

• He should/ought to be home by now. He is always home before 7. 
• They should/ought to arrive before midnight.

We use should have/ought to have + past participle to talk about a situation expected to happen in the past. We can also use this form to express criticism.

• He should/ought to have received the parcel yesterday. I sent it two days ago. 
• He should/ought to have studied more. Nobody fails if they study. 

Expressions for speculation

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Bound/sure

Bound and sure are adjectives used to say that we are quite sure that something will be true or happen. We use them in the structure be bound/sure + to-infinitive.

• The Jamaican is bound/sure to win the final.
• There are bound/sure to be some discrepancies during the meeting, so be prepared.

Likely/unlikely

Likely and unlikely are adjectives. If something is likely to happen, it means that it will probably happen or is expected to happen. If something is unlikely to happen, it means that it probably won’t happen. We can use these two adjectives in two ways.

It is likely/unlikely that + clause

• It’s likely that just in a few years, we will change our economic model.
• It’s unlikely that the police had anything to do with it. 

Subject + be likely/unlikely + to-infinitive

• He is likely to win this game. 
• They are unlikely to reach an agreement. 

Definitely/probably

Definitely and probably go before the main verb and after the auxiliary verb (if there is one) in positive sentences.

• You’ll definitely get the job. Nobody’s got your qualifications and experience. 
• He’s definitely our best player. 

And they go before the auxiliary verb in negative sentences.

• They definitely won’t find any evidence. 
• She definitely isn’t seeing anyone right now.