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
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 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.
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)