What are the rules for using past tense modal verbs like could have?
When we want to talk about a hypothetical (i.e., imaginary, or proposed) past, we use modal verbs like could have, should have, or would have, followed by a past participle (also known as the “third form”). Below are situations when we would use each of these.
Using Could have + past participle
- When we want to describe something that we could have done or was possible to do in the past, but we did not do, we use could have + past participle. For example:
- I could have bought milk on the way home, but I forgot.
- She could have passed the test if she had studied harder.
(also see Modals of Ability)
How do I make the negative of “could have”?
If something wasn’t possible in the past even if we had wanted to do it, we use couldn’t have + past participle. For example:
- I couldn’t have made it to the store because it was already closed when I left work.
- He couldn’t have paid for the groceries because he left his wallet at home.
We also use could have + past participle when we want to make a guess about something that happened in the past. Of course, since we’re guessing, we don’t know whether we’re right.
- She could have gone to the wrong room.
- I could have left the water running.
Note that might have + past participle can be used in the same situations:
- She might have gone to the wrong room.
- I might have left the water running.
(also see Modals of Probability)
Using Should have + past participle
- When we refer to something that would have been a good idea or good advice, but we did not do it, we use should have + past participle. This is usually used to express regret. For example:
- She should have taken the motorway to get here on time.
- He should have eaten a healthier diet to prevent high cholesterol.
- I should have remembered our anniversary!
- When we want to refer to something that we assume would happen if everything was as we expect it, we use should have + past participle. In these situations, we are not entirely sure that everything has gone as we expect. For example:
- The package should have reached her by now.
- He should have arrived at work by now.
We can also use should have + past participle to describe that would have happened if everything was expected, but clearly hasn’t happened:
- The package should have reached me but it hasn’t.
Using Would have + past participle
- We use would have + past participle as part of the third conditional:
- If I had left work sooner, I would have been home by now.
- We can also use would have + past participle to describe something that we wanted to do but did not. The difference between this and the third conditional is the absence of an “if clause.”
- I would have told you sooner, but you were busy. (note: This could also be phrased as “If you had not been so busy, I would have told you sooner.”)
- She would have helped you, but she had other plans. (or, “If she had not had other plans, she would have helped you.”)