How to use modal verbs for probability, speculation & deduction

How do we express probability in English? How do we use speculation? How do we make a deduction?  Find out about probability, speculation & deduction using modal verbs in English in this video lesson...


A very common use for modal verbs in English is to express probability. We can express probability as speculating or making a deduction. This means expressing how much we believe something is true or expressing how possible we think something is. We're saying how much we believe something or not. So, for example..."The universe is so big that there must be aliens somewhere" So, I don't have evidence, I don't have proof, I don't know for sure, but in this case I'm speculating, I'm making a deduction. In this case I'm using the modal verb 'must' to express that I'm almost 100% sure that something is true.

Now if you think something is not true and you're very sure that it's not true, you can use 'can't', for example..."My friend says he met an alien but he's always lying so this can't be true". So again I'm 100% sure, but in this case we're expressing something in the negative. So the opposite of 'must' is 'can't', not 'mustn't'.

If you want to say that you're only 50% sure, you could use 'could' or 'may' or 'might'. You could say: "it could be true", "it may be true" or "it might be true", but in these examples I'm only 50% sure. As I mentioned before, we can use 'will' to express a high probability, so if you're very sure about something, you can also use 'will'. So for example if the phone rings, you can say: "That will be for me". That means that you're very sure, you know who is calling. If you're not so sure, you're not 100% sure, maybe 75% sure, you could use 'should'. You could say: "That should be for me". So I'm quite sure, but I'm not 100% sure.

It's also really important to be able to express probability in the past. So imagine you're a detective trying to solve a crime. You would need to speculate or to deduce what happened in the past. So for example: "The thief must have known there was money in the house". So to express probability in the past we're using 'must' plus 'have' plus the past participle. In this case 'must have known'.

If you want to express probability in the negative, but express that you're very sure, you could say for example: "The thief can't have used a key because the window was broken". Again, you're 100% sure, but you're expressing probability in the negative. This is 'can't' plus 'have' plus the past participle.

If you're only 50% sure, then you would use 'might' plus 'have' plus the past participle. "Someone might have told the thief about the money in the house" but in this case I'm only 50% sure, I'm speculating, I'm guessing based on the information I have. And instead of 'might' you could use 'may' or 'could'. "Someone may have told the thief" or "Someone could have told the thief". All three mean the same. So in all examples here, we use the modal verb plus 'have' plus the past participle to express past probability.

As before you could use 'will' to express that you're 100% sure or you could use 'should' to express that you're 75% sure. "The thief will have arrived at the airport by now, so the police should have arrested him". This means I'm sure the thief has arrived at the airport at this time, so I'm quite sure that the police have had the chance to catch him at the airport.

It's really important to be able to express probability, both in present and the past, because we spend a lot of time in English talking about the things that we...that we know or the things that we believe, both in the present and in the past, so it's really important to practise talking about probability, speculating about things that we know or expressing a deduction based on the information we have. So in order to practise that, what do you believe? What do you think? What do you know? Put your ideas in the comments below. Use this language to express probability. The more you practise it, the more you'll understand it. If you're not so sure yet then go back and watch the video again. The second time it will be much easier!

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