MODALS OF DEDUCTION
Grammatical form
subject + modal auxiliary verb + verb bare infinitive
subject + modal auxiliary verb + be + verb-ing
It must be the postman.
She might be having a bath.
subject + modal auxiliary verb + have + verb past participle
subject + modal auxiliary verb + have + been + verb-ing
They could have got in through the window.
subject + modal auxiliary verb + be + verb-ing
It must be the postman.
She might be having a bath.
subject + modal auxiliary verb + have + verb past participle
subject + modal auxiliary verb + have + been + verb-ing
They could have got in through the window.
General use of modal verbs of deduction

We can make deductions about the present :
'Where's Joanna?'
'She's not here. She must be in the kitchen.'
We can also make deductions about the past :
'How do you think the burglar got in?'
'He must have climbed through the window.'
We use different modal axiliary verbs depending on how strong our guess is :
He must be in the kitchen. - Very sure - 99%
He can't be in the garden. - Very sure - we think it's impossible - 99%
He may / might / could be in his bedroom. We think it's possible - 50%
to make guesses about the past, we use the same modals and add the auxiliary verb have. This is followed by the past participle of the main verb :
He must have climbed through the window. - Very sure - 99%
He cant't have climbed through the window. - Very sure - we think it's impossible - 99%
He may / might / could have climbed through the window. We think it's possible - 50%

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