We can use these modal verbs (also called modals of deduction, speculation or certainty) when we want to make a guess about something. We choose the verb depending on how sure we are.1: Talking about the present:
must / might / could / may / can’t
must + infinitive
might / might not + infinitive
could / could not + infinitive
may / may not + infinitive
can’t + infinitive
For example:I am waiting for Julie with another friend, David.
I ask: ‘Where is Julie?’
She must be on the bus. (I’m fairly sure this is a good guess)
She might come soon. (maybe)
She could be lost. (maybe)
She may be in the wrong room. (maybe)
She can’t be at home. (I’m fairly sure this isn’t true)
Notice that the opposite of ‘must‘ is ‘can’t in this case.
Will / won’t
We use will and won’t when we are very sure:
She’ll be at work now.
Should / shouldn’t
Should and shouldn’t are used to make an assumption about what is probably true, if everything is as we expect:
They should be there by now.
It shouldn’t take long to drive here.
This use of should isn’t usually used for negative events. Instead, it’s a better idea to use will:
The underground will be very busy now (not: ‘should be’).
Can is used for something that is generally possible, something we know sometimes happens:
Prices can be high in London.
Can is not used to talk about specific possibilities:
He could be on the bus (not: ‘can be’).
2: Using modal verbs to talk about the past:
must / might / could / may / can’t + have + past participle
must have + past participle
might / might not have + past participle
could / couldn’t have + past participle
may / may not have + past participle
can’t have + past participle
For example:You: Where was Julie last night?
She must have forgotten about our date.
She might have worked late.
She could have taken the wrong bus.
She may have felt ill.
She can’t have stayed at home.
Will / won’t + have + past participle
Will and won’t / will not + have + past participle are used for past certainty (compare with present use of ‘will’ above):
The parcel will have arrived before now.
Should + have + past participle
Should + have + past participle can be used to make an assumption about something that has probably happened, if everything is as we expect (compare with present use of ‘should’ above):
The train should have left by now
We can use could + infinitive to talk about a general possibility in the past (compare with the use of ‘can’ above):
Prices could be high in the sixteenth century.
This is not used to talk about specific possibilites in the past (instead we usecould + have + past participle):
He could have been working late (not: ‘could be’. As this is a specific possiblity, ‘could be’ is present tense)