When we want to talk about a hypothetical (i.e., imaginary, or proposed) past, we use 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.
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)
Making 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)
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.
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.”)