Grammar Time: Modals of Deduction

Modals of deduction about the present

We can use modal verbs for deduction – guessing if something is true using the available information. The modal verb we choose shows how sure we are about something. Let’s learn how to use modals of deduction to say how certain we are about a possibility.

Deduction – the process of using the knowledge or information you have in order to understand something, form an opinion, guess or draw a conclusion about the facts.

Depending on the information available, you might be more certain that your conclusion is true, or less certain that your
conclusion is true – and we use different modal verbs to indicate the degree of certainty.

Exam in Mind Level B1/B2/C

The structure of all modals of deduction (present) is the same:

  • Affirmative: Subject + modal + V1 (bare Infinitive)
  • Interrogative: Modal + subject + V1 (bare Infinitive)?
  • Negative: Subject + modal + not + V1 (bare Infinitive)

Note that these verbs, like all modal verbs, are followed by an infinitive without to.

You are 100% sure (completely or almost certain)

must / have to

We use must / have to when we feel sure that something is true because there’s very strong evidence or it’s the only realistic possibility.

  • This must be her house. I can see her car in the garage.
  • He must live near here because he always walks to work.
  • Come inside and get warm. You must be freezing out there!
  • He must live near here because he comes to work  on  foot. (We don’t know where he lives but we’re sure it’s not far away)
  • Are you a computer programmer? It must be a well-paid job.
  • He has to be in Colorado Springs by now. It takes an hour to drive there, and he left three hours ago. 

The negative for must or have to is can’t or couldn’t:

  • He can’t be at the hotel yet. His plane just landed 10 minutes ago. 
  • He couldn’t be in Colorado Springs yet. It takes an hour to drive there, and he only left 15 minutes ago. 

Modal verbs of 80% expecting to be certain

should / ought to

  • He should be at the hotel by now. His plane landed two hours ago. (You are not giving him advice. You are mostly sure than his plane landed on time, and the hotel is 30 minutes away form the airport. So unless there is a delay, you have figured out that he is likely to be at the hotel).
  • He shouldn’t be at the hotel yet. His plane only landed 45 minutes ago.
  • She should be on her way to work now. She is never late. (You know that she is never late. You also know that it is almost time for work, so you expect her to be in her car driving).
  • The package I sent ought to be there by now. I sent it two weeks ago. (You know that it only takes one week for a package to arrive, so you are expecting it to be there by now).

You are 50% sure (maybe certain)

might/ may / could

We use mightmay or could to say that we think something is possible but we’re not sure. They all have the same meaning, but may is more formal than might and could.

  • She’s not here yet. She might be stuck in traffic.
  • He’s not answering. He could be in class.
  • We regret to inform you that some services may be delayed due to the bad weather.

The negative of these modal verbs are may not, might not, and couldn’t.

You are 100% sure it’s not possible


We use can’t when we feel sure that something is not possible.

  • It can’t be far now. We’ve been driving for hours.
  • She can’t know about the complaint. She’s promoted him to team leader.
  • It can’t be easy for him, looking after three kids on his own.

Do this exercise to test your grammar concerning modals of deduction.

1.Do you remember my birthday party last year? You _____ remember! It was the same day as your graduation.

a. can’t
b. could
c. must

2. Come in and sit down. You _____ be tired after the journey.

a. must
b. might
c. can’t

3. Whose is this coat? It _____ be Paul’s. It’s way too small for him.

a. must
b. could
c. can’t

4. The exam results will be out soon. It _____ be this week or possibly next week.

a. must
b. might
c. can’t

5. She always gets such good grades at school. She _____ work very hard.

a. must
b. could
c. can’t

6. The doctor said my headaches _____ be because of the hot weather but they’re going to do some tests to be sure.

b. could
c. can’t

7. He _____ be our teacher! He looks about twelve!

a. must
b. might
c. can’t

8. Take this umbrella just in case. It looks as if it _____ rain.

a. must
b. might

9. The house isn’t hard to find. It’s the red one at the end. You _____ miss it!

a. must
b. might
c. can’t

10. What an amazing trip! You _____ have some incredible photos.

a. must
b. might
c. can’t

11. That _____ be the vegetarian option. It’s got chicken in it.

a. must
b. may not
c. can’t

12. Have you got your passport? I’m not sure if you’ll need it but they _____ ask you for ID.

a. can’t
b. might
c. must

13. Who left their laptop on my desk? It _____ be Mel’s – she’s working at home today.

a. must
b. could
c. can’t

14. Samira has flu. We don’t know yet but she _____ need to take the whole week off.

a. must
b. can’t
c. may

15. Your watch says a different time from mine. One of them _____ be wrong.

a. must
b. could
c. may

16. Look at that bird! Maybe it’s an eagle or it _____ be a vulture.

a. must
b. could
c. can’t

Modals of deduction about the past

The structure of modals in the past is different, but the meaning is quite similar (just refers to the past):
modal verb + have + V3

must  +  have + V3 – we are sure that something was true; we draw the conclusion that something DID happen:

  • Where is my wallet?! Someone must have stolen it!
  • You’re soaked. It must have been raining outside.
  • I’m sorry but Mr White isn’t here. He must have left.
  • Sheila got a tan. She must have spent a lot of time in the sun lately.
  • There was one banana left, but now it’s gone. My husband must have eaten it.

must not have + past participle – when we draw the conclusion that something did NOT happen:

  • The car is still dirty. Paul must not have washed it yet.
  • He barely touched his lunch. He must not have been hungry.

should have / ought to have + V3 – we use it to talk about a situation that we expected to happen in the past, but it didn’t. Or something we expected not to happen and it happened. We often use this form to express criticism.

  • We really enjoyed the film. You should have come with us.
  • You shouldn’t have broken up with her. She was perfect for you.
  • You should have told me about the sale. I could have got some new shoes at a discount!
  • I sent the package three weeks ago with express mail. They should have received it already.(= I expect that they have already received it)
  • This car is brand new. It shouldn’t have broken down. (= I expected it NOT to break down)

Have practice

Think of one thing you regret doing, and one thing you regret NOT doing. Make sentences about them using should/shouldn’t have + the past participle:

I should have…
I shouldn’t have..

might, may, could + have + V3 – we think something was possible but we aren’t sure:

  • The thieves might have escaped by car but we can’t be sure.
  • He should be hour by now. He may have been delayed by a traffic jam or something.
  • I can’t find my purse. I could have left it in the supermarket but I just don’t know.
  • My car isn’t there any more! It might have been stolen while I was shopping.
  • We may not have bought enough beer for the party.

may have (possible 50%) – I suppose the internet may have changed the way we read forever.
might have (less certain 30%) – It’s possible that e-readers might have changed our reading habits permanently.
could have (less certain 30%) – The traditional newspaper could have become a thing of the past.

can’t, couldn’t + have V3 – we feel quite sure that something did not happen in the past or it was not true in the past, we are certain that something was impossible (o – 1% certainty):

  • I thought I saw John in town this morning but it can’t have been him – he’s in Greece this week.
  • I can’t have left it in the supermarket – I had it on the bus on the way home.
  • You can’t have read the instructions properly. They’re perfectly clear.
  • Martha couldn’t have taken your notebook; she wasn’t even in class yesterday.
  • The cookies are gone. But Eric couldn’t have reached the cookies on the top shelf; he must have asked his older brother to get them.


  • Can’t have is much less common than couldn’t have.
  • The difference between must not have and couldn’t have:
    I don’t see the report here – she must not have printed it out. (we draw the conclusion that she did not do it)
    The printer’s been broken for the past week, so she couldn’t have printed out the report. (we know it was IMPOSSIBLE for her to do it)

Have practice using modals of deduction about the past.

Choose the correct modals of deduction in these sentences.

  1. I don’t know why there weren’t any buses yesterday. They could have been / must have been  on strike but I’m not really sure.
  2. You could have seen / must have seen her. She was standing right next to you.
  3. We’ll ask at reception. Someone might have handed / must have handed your keys in.
  4. He can’t have gone / mustn’t have gone to France. He hasn’t got a passport.
  5. I explained it but she may not have misunderstood / may have misunderstood – my Japanese isn’t very good.