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Past Modals for Degree of Certainty

We can use past modals to  talk about how sure we are that something happened.

The general form for using past modals is:

modal + have + past participle

The charts below demonstrate how to correctly use past modals.

 

Question:  Why didn’t Mike come to lunch with us?

Degree Example sentence Comment

FACT

He had a meeting with a client. The speaker is 100% sure. This is a fact.

Almost 100% Sure

He must have had a meeting with a client. The speaker is making a logical conclusion and is almost sure (but not completely sure) that Mike had a meeting with a client. If Mike always goes to lunch with the speaker unless he has a meeting with a client, then it would be reasonable to say that he must have had a meeting with a client.

Possibility

He may have had a meeting with a client.
He might have had a meeting with a client.
He could have* had a meeting with a client.
The speaker is simply mentioning a possibility here. All three examples in the first column are ways of saying “It’s possible that Mike had a meeting with a client.”

*If there was a possible result that we know didn’t happen, we use could have.
Example: You’re lucky you didn’t get in trouble for coming late without calling. You could have been fired. (In this example, we know that the person did not get fired. It was a possible result in the past that did not happen.)

 

Question:  Why didn’t Mike come to lunch with us?

Degree Example sentence Comment

FACT

He wasn’t in the office. The speaker is sure that Mike was not available. This is a fact.

Impossible

He couldn’t have been in the office. The speaker believes that it was impossible for Mike to have been in the office. Maybe the speaker spoke to Mike right before lunch and Mike said he was 100 miles away visiting a client.

Almost 100% Sure

He must not have been in the office. (not mustn’t have been)** The speaker is making a logical conclusion. The speaker is 95% sure that Mike was not in the office.

Possibility

He may not have been in the office.
He might not have been in the office. (not mightn’t have been)
The speaker is simply mentioning a possibility here. All three examples in the first column are ways of saying “It’s possible that Mike was not in the office.”

**Okay in British English but not common in American English

 

Reductions of Past Modals

When we speak, we typically reduce the past modals above. See the chart below.
Note that we never write the reduced form. The second column simply shows what the words sound like.

Past Modal Reduction (what it sounds like in American English)
must have must’ve
may have may’ve
might have might’ve
could have could’ve
couldn’t have couldnt’ve
must not have must not’ve
may not have may not’ve
might not have might not’ve

To practice, try this past modals practice exercise.