Modal Verbs for Deduction

We can use modal verbs to talk about probability or improbability of something, as well as to make deductions about some facts or events.

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

Modal verbs for certainty

Certainty about the present

— To speak with certainty about things in present, we use ‘must’:

• John must be very tired, he’s been staying in bed all day long.
• They must be absent, I cannot find them anywhere.

— To speak about something you consider absolutely impossible, use ‘cannot (can’t)’:

• Jack cannot be at school, I saw him playing basketball at the playground.
• Pete’s leg can’t be broken, he walked to the doctor without any help.

Certainty about the past

To speak about something that you think definitely happened in the past, use  ‘must have’ with the Past Participle:

• I can’t find my socks. My wife must have taken them to the laundry room.
• Nobody opens the door, my friend must have left the house already.

— To speak about something you are absolutely sure did not happen in the past, use ‘couldn’t have’:

• I saw a flash in the sky, but it couldn’t have been aliens, they don’t exist.
• Paula couldn’t have been at the party last night, she was at work.

Modal verbs for uncertainty

Uncertainty about the present

— To talk about something with uncertainty, use modal verbs ‘might’ and ‘could’:

• Sarah isn’t feeling well. She might have a cold.
• I have a high temperature. It could be a flu.

— To describe negative things you are not certain about, use ‘might not’:

• Your leg is not swollen, so it might not be serious.
• I can’t reach Tom by phone, he might not be in the city.

Uncertainty about the past

In case you are not sure whether something happened in the past, use ‘may have’, ‘might have’ or ‘could have’ instead of ‘must have’:

• Cindy didn’t return my call yesterday. She might have forgotten to call me back.
• I didn’t see who knocked on the door, but it may have been the postman.

Check out this short video from Learn English with KT to understand the difference between ‘must’, ‘can’t, ‘could, and ‘might’ for making deductions:

Read more on modal verbs:

Modal Verbs: Overview

Modal Verbs for Possibility

Modal Verbs for Obligations