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Hello, this is Victoria Moriejo. Today we are looking at deduction in the past.

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The modal verbs of deduction about the past are the following.

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Must have, might have, may have, can't have, couldn't have.

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How do we know if it's a deduction about the present or about the past?

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Look at the examples.

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She must be really tired from working all weekend.

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This is a deduction about the present, because the infinitive is a simple infinitive, the verb to be.

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Look at the second example.

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She must have been very tired after she had to work all weekend.

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This is a deduction about the past, because after must we have have been, which is a perfect infinitive, or a past infinitive.

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So this is a deduction about the past.

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When we are looking at deductions, depending on the model that we choose, we express different levels of certainty.

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For example, if we are using must plus have plus past participle, we are expressing total certainty.

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Helen couldn't find her phone. She must have left it at the school.

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we are very sure that this is true. Rob must have had a good weekend. He's been

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smiling all morning. We are quite confident that this is true. Then Mike

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must have saved a lot of money. He's going on holiday to the Caribbean. Again

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certainty. If we are not so certain we use a different modal verb. We use might

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or may have plus past participle this is used to express possibility something

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that something happened it may have happened or it may have not for example

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Helen couldn't find her phone she might have left it at the school or Rob may

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may have had a good weekend or Mike may have saved a lot of money is a bit

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Then, when we think something is impossible, we can use the modal verse CAN'T or COULDN'T plus have plus past participle.

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Right? This means that something is not possible.

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For example, Rob can't have had a good weekend. He's had a long face all morning.

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Or Mike couldn't have saved a lot of money. He's always eating out and shopping online.

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Or Helen can't have left her phone at school. She didn't take it there.

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That's all. Thanks for listening. Do you have any questions? Let me know. Bye bye.