3.2: Past Modals of Degrees of Certainty

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Allyson Marceau
San Jacinto College

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Modals of Possibility and Probability for Past Situations

Must, may, might, can’t and couldn’t are used with a present perfect verb form to show how certain a speaker is that a past situation happened or didn’t happen.

Affirmative

Formula: MODAL + HAVE + PAST PARTICIPLE (to show how certain the speaker is that something happened)

Must have + past participle shows a great deal of certainty that something happened, but there is still some doubt in the speaker’s mind. He/she is not 100% certain. For example:

The students did well on the test. They must have studied hard.

(There are other possible reasons why the students did well on the test. It might have been very easy, or they might have already learned the material. The speaker uses must have studied hard to show that he/she thinks this is the most logical reason they did so well.) For example:

I’ve called Jim several times, but he doesn’t answer. He must have turned off his phone.

(The speaker believes this is the most logical reason why Jim doesn’t answer the phone. However, other reasons are possible. He might not have heard the phone, or he might not have wanted to talk to anyone.)

May, might and could convey the idea that a speaker is only 50% or less certain that something happened. The use of could implies a little more certainty. For example:

My wife wasn't at home when I arrived. She may (might, could) have gone shopping.

(The speaker doesn’t know for sure that his wife went shopping. However, it is his best guess.)

Negative

Formula: MODAL + NOT + BASE VERB + PAST PARTICIPLE (to show how certain the speaker is that something didn’t happen)

Can’t or couldn’t have + past participle shows that the speaker is very certain something didn’t happen. For example:

You couldn't have seen Susan at the party. She stayed home last night.

(The speaker is certain that the listener didn't see Susan because she knew Susan wasn't there. Use contractions, not could not or cannot in this context.)

May or might not have + past participle shows that the speaker is 50% or less certain that something didn’t happen. For example:

There was a strange light in the sky last night. Leon was sure it was a plane, but I'm not so sure. It may not have been a plane. It might have been a UFO (Unidentified Flying Object).

An appropriate modal + have is used in most short answers. However, if the question has a be verb, the formula for responding is appropriate modal + have been. For example:

Person A: Has Jennifer decided which courses she’s going to take next semester?

Person B: I’m not sure. She might have.

Person C: Do you think Tim enjoyed the concert?

Person D: I don’t know. He may have.

Page from: ELC Study Zone by University of Victoria's English Language Centre is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License Edited for currency and region.

Also see Might / Must have – past guesses and inferences

A Note about Must:

As mentioned above, Must can be used to express strong certainty about the present or past:

You must be tired. You worked very hard today.

Carlos must have studied hard. He got an "A" on the test.

Must can also be used to express necessity or obligation:

Students must study in order to pass their classes.

In this meaning, the past is expressed with had to:

Carlos had to study for his math test last night.

Exercise 3.2.1: Rewrite the Sentence-Past Modals

Rewrite the sentences with the choices given.

Example: Perhaps she jumped

A. may have

B. could have, could've

Answer
A. She may have jumped. B. She could have jumped.

1. Maybe he said that. She can't remember now.

A. might have
B. could have

A. ___________________________ B. ___________________________

2. Perhaps she saw us.

A. could have
B. might have

A. ___________________________ B. ___________________________

3. I'm pretty sure we took the wrong street.

A. must have