In this final part of my modal verbs series, I’d like to address modals of probability (or deduction). We use modals to talk about the chance or probability that something will or will not happen in the future. We call them degrees of probability.

Blog_Modal Verbs of deduction

Source: The EFL SMART Blog

The table below gives you an overview which I will develop.

100% CERTAINTY Will, be certain to
95 – 100% DEDUCTION Must, can’t
80% EXPECTATION Should, shouldn’t, ought to, ought not to, be likely to, be unlikely to
30 -70% UNCERTAINTY May, may not, might, might not, could

When we are certain that something will happen we use will and be certain to.

  • I will present the document at the next AGM.
  • They are certain to introduce the new prototype now that they have had approval.

When we are certain that something will NOT happen, we use won’t

  • I’m sorry, James is on holiday. He won’t be back until the end of the month.

If we want to show that something is certain because it is logical form the evidence we use must or can’t. This is called “deduction”. Note that can’t is used and not mustn’t.

  • Carl is not answering? He must be home. I only just spoke to him on the home phone number.
  • Jessica is not answering her phone. She must be out.
  • Cecile can’t have arrived home already. She left the office two minutes ago. (The journey from the office to her house takes 20 minutes)


Source: The English Blog

Source: The English Blog

When we expect something to happen we use should, ought to, be likely to

  • The flight should arrive on time this evening
  • Our profits are likely to improve this year
  • It ought to be a better year for us now that we have made the structural changes.
    (NOTE: “ought to” is used formally and is not as natural in spoken English as “should” or “be likely to”.)

If we don’t expect something to happen, we shouldn’t, ought not to, be unlikely to

  • I have all the information I need, so preparing the report shouldn’t take long.
  • She hasn’t responded to my calls today so the meeting is unlikely to happen today.
  • There ought not to be a problem anymore with the server as IT have been working on it all day.

May, might or could are normally used for when we are uncertain that something will happen. The meaning is “perhaps” or “maybe”.


  • I might be able to fit you in for a lesson next week.
  • I may have to change the booking at the last minute.
  • It could take a while to find a taxi at this hour.


The negative forms are may not or might not. “Could not is not used with this meaning.  It’s used with past ability. (See Part 1 of this series)

  • I have a suggestion that you may not/ might not agree with.
  • You may not want to go out this afternoon if it rains.



Structure: Modal verb + have + past participle

  • You’ll have seen the papers recently. We’re all over the news. (Certainty)


  • There was no answer from her phone. She must have been in a meeting. (Deduction)
  • The fridge was full yesterday. They can’t have eaten all the food. (Deduction)
  • They should have been here by now. I hope there isn’t a problem. (Expectation)
  • We’re only five minutes late. The meeting might not have started yet. (Uncertainty)
  • You won’t have seen the latest edition yet. It’s not out. (Certainty)


And there you have it. For more practice on modals of deduction, take a look at this excellent blog post by fellow teacher David Mainwood of The EFL SMART Blog.

Click on Part 1 & Part 2 if you missed them. I hope you found the series on modal verbs a useful refresher.

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Ciao for now.


Business Grammar Builder, Paul Emmerson (Macmillan) 2010


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