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Past modals of deduction

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To make guesses or deductions about past actions or states use a past modal of deduction. This is formed with a modal verb + have + past participle also called the perfect infinitive.

Where's my phone?
You could have left it in your office.

To make deductions about continuous actions or states use modal verb + have + been + ing form.

Why didn't Sarah come to the party last night?
She must have been feeling ill.

For deductions using the passive use modal verb + have + been + past participle.
Your phone could have been stolen from your jacket.

Usage of different modals:
must have – you are almost certain that something is true based on some evidence.
I must have left the window open. That's how the cat got in.

might/could/ may have – you think it's possibly true based on evidence.
They could have been delayed by rain. It does rain heavily in the mountains this time of year.

Couldn't/ can't have – you are almost certain that this is true or impossible based on evidence.
He can't have left the office yet. It's only 4 o'clock. He must be in a meeting.

Lesson by Tristan, English teacher at EC Malta English school

Choose the correct form for the following:

  • 1) A: Look at her diamond ring. B: Yes, it __ cost a fortune.

  • 2) A: Patrick isn't answering his phone. B: He __ switched it off or left it at home.

  • 3) A: Where's Sarah? B: She said she had an important meeting tomorrow. She __ working late.

  • 4) A: Peter said you were an architect. B: He __ confused me with someone else or maybe he misunderstood me. I'm a vet.

  • 5) A: I can't find my wallet. B: You __ looked properly. I put it on the table myself.

  • 6) A: Does John know about the party? B: I suppose so. He __ have been told by Sarah but then again maybe not.