Modal Verbs of Probability

1.Use of Modal Verbs of Probability

We can use these modal verbs when we want to to say how sure we are that something happened / is happening / will happen. We choose the modal verb depending on how sure we are.

2. Talking about probability in the present:

must / might / could / may / can’t + infinitive

For example:

Sitting at home and the phone rings John guesses:

That must be Susan she said she would call about this time. (I’m fairly certain it is her.)
It might be a wrong number. (maybe)
It could be my brother, he hasn’t called in a while. (maybe)
It may be my boss. I didn’t go to work today. (maybe)
That can’t be Alan he is flying to America at the moment. (It is impossible)

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 the airport by now.
He won’t be here for a few hours, he has called and said he is delayed.

Should / shouldn’t

Should and shouldn’t are used to make an assumption about what is true or will be true in the future, and to show you have reasons for your suggestion:

It’s nearly six o’clock. They should be here soon.
The traffic is fine it shouldn’t take more than an hour to get here.

This use of should isn’t usually used for negative events. Instead, it’s a better idea to use will:

There will be a lot of traffic (not: ‘should be’).

Can

Can is used for something that is generally possible, something we know sometimes happens:

It can be very cold in Scotland in winter.

Can is not used to talk about specific possibilities:

He could be on the train (not: ‘can be’).

3. Using modal verbs to talk about probability in 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: Susan didn’t arrive for a meeting last night. John thinks about the possibilities:

She must have forgotten about our meeting.
She might have worked late.
She could have got lost.
She may have felt ill.
She can’t have stayed at home, she always goes out on Fridays.

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 plane will have landed by 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):

It’s nearly ten o’clock. They should have arrived by now.

Could

We can use could + infinitive to talk about a general possibility in the past (compare with the use of ‘can’ above):

Life could be hard in the 19th century.

This is not used to talk about specific possibilities in the past (instead we use could + have + past participle):

She could have been waiting for me at the wrong place. (not: ‘could be’. As this is a specific possibility in the present tense)