Modals for deduction and supposition – Past

Look at this picture. We know nothing about these people but we can make guesses. When we make guesses about people, we call this deduction and supposition.

When we are certain something happened in the past, we use the verb: must have.

For example: They must have lost their keys.

(I don’t know for sure, but I’m 99.999% certain).

When we are certain something wasn’t true or didn’t happen, we use: can’t or couldn’t have.

For example: They can’t/couldn’t have remembered to take their keys.

(I don’t know for sure, but I’m 99.999% certain).

When we are not sure but we think something was possibly true, we use: may (not) have, might (not) have, could have. We DO NOT use could not have in this case.

For example: They may/might/could have left their keys at work.

For example: They may not /might not have lost their keys.

Click on the links below to learn more about modal verbs

Introduction

Modal Verbs for Ability

Modal Verbs for Prohibition

Modals for Permission

Modals for Offers and Requests

Modals for Suggestions and Advice

Modal Verbs for Prediction

Modals for deduction and supposition – Present

Modal Verbs for Obligation and Necessity

Past Modals