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.