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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 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)