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.

 

 

General use of modal verbs of deduction

We can use certain modal auxiliary verbs to make deductions, i.e. to make guesses based on known facts.

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%

See also functions : making guesses

 

Note

When using modal auxiliary verbs for deductions, the opposites of

must and must have are can't and can't have,

not mustn't and mustn't have. 

Similarly, we don't use can or can have for making deductions.