THE MODAL VERBS OF PROBABILITY

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 + infinitive

For example:
-I am waiting for Julie with another friend, David.
-I ask: 'Where is Julie?'
-David guesses:

+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:

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 ask: Where was Julie last night?
-David answer:

+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.

Could:

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 possibilities in the past (instead we use could + have + past participle):

He could have been working late (not: 'could be'. As this is a specific possibility, 'could be' is present tense).