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.
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 might, may 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.