Modals of deduction

We use the modal verbs might, may and could + an infinitive form without to when we think it is possible that something is true. We use must when we are sure it is true and can’t when we are sure it isn’t true.

A: So we can use might, may and could for making guesses?

B: Yes. They’re all used when we are not sure about something, but we think that it is possible that it is true.

They might be some kind of small pig.

A: But could I also say, ‘They could be a kind of pig’ or ‘They may be a kind of pig’?

B: Yes, you could. The meanings are really similar. You can choose whether to use may, might or could.

A: Ah, OK. So if I’m not sure about something I can use may, might or could and the meaning is almost the same. That’s easy to remember. 

B: Yes, that’s right. But there’s something else which affects the meaning, and that’s how you say it; the ‘intonation’. If you stress may, might or could it sounds less probable.

It could be mango juice.   (less likely)
It could be mango juice.   (more likely)

A: OK, and if I’m sure about something, what modals do I use?

B: We use must if we think something is true and can’t if we think something isn’t true.

So, it must come from an animal.
That looks like tomato juice, but it can’t be, that would be too easy.

A: Do the verb forms change in the third person?

B: No, they don’t change. They are the same in the first, second and third person.

A: Great! I think I like these modals!

B: You’re right. Once you know the rules, they’re pretty easy. Don’t forget they are always followed by the infinitive form without to. We also use may/might + be + -ing for something happening right now or a possible future arrangement.

That might be Mum phoning from Bali.
She might be going to Australia.

You also used ‘it could be a kind of pig‘ for describing something in the present.

Yes, but that was with the verb to be, a stative verb. We use the –ing form with active or dynamic verbs.

A: I see. Is there anything else to be careful about?

B: Yes, if you are talking about possibility in the past you need the modal + have + past participle.

She might have taken those photos in China.
I may have thrown it away by mistake.

A: Hmm. I think I may need to practise these verbs a bit.

B: Yes, you might be right!

Modals: deductions about the present

Grammar explanation

We can use modal verbs for deduction – guessing if something is true using the available information. The modal verb we choose shows how certain we are about the possibility.

must

We use must when we feel sure that something is true or it’s the only realistic possibility.

This must be her house. I can see her car in the garage.
He must live near here because he always walks to work.
Come inside and get warm. You must be freezing out there!

might, may, could

We use might, may or could to say that we think something is possible but we’re not sure. 

She’s not here yet. She might be stuck in traffic.
He’s not answering. He could be in class.
We regret to inform you that some services may be delayed due to the bad weather.

They all have the same meaning, but may is more formal than might and could.

can’t

We use can’t when we feel sure that something is not possible.

It can’t be far now. We’ve been driving for hours.
She can’t know about the complaint. She’s promoted him to team leader.
It can’t be easy for him, looking after three kids on his own.

Modals: deductions about the past

Grammar explanation

We can use modal verbs for deduction – guessing if something is true using the available information. The modal verb we choose shows how certain we are about the possibility. 

must have

We use must have + past participle when we feel sure about what happened.

Who told the newspapers about the prime minister’s plans? It must have been someone close to him.
The thief must have had a key. The door was locked and nothing was broken.
Oh, good! We’ve got milk. Mo must have bought some yesterday.

might have / may have

We can use might have or may have + past participle when we think it’s possible that something happened. 

I think I might have left the air conditioning on. Please can you check?
Police think the suspect may have left the country using a fake passport.

May have is more formal than might have. Could have is also possible in this context but less common.

can’t have / couldn’t have 

We use can’t have and couldn’t have + past participle when we think it’s not possible that something happened.

She can’t have driven there. Her car keys are still here.
I thought I saw Adnan this morning but it couldn’t have been him – he’s in Greece this week.

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