We can use modal verbs to talk about how sure or unsure we are about something in the past just as we use modals in the present with a slight change in the form.
- He must be really happy about his promotion. (present deduction)
- He must have been very happy when he was told about his promotion. (past deduction)
When we use a modal verb to talk about a situation where we are not expressing a fact but we are using deduction the form is MODAL + have + past participle (verb 3)
Must have + past participle
We use ‘must have + past participle’ when we are quite sure about something.
- You must have been very pleased when you received the results of your exams.
- He must have forgotten his phone at home again. He’s not answering.
- I must have left my keys in the car. I can’t find them.
Might have/may have/could have + past participle
We use ‘might have/may have/could have + past participle’ when we are not sure about something but we think it was possible.
- He was supposed to be here an hour ago but he could have been stuck in a traffic jam.
- He may have said he was coming but I can’t really remember. I wasn’t listening.
- I might have been here when I was a child but I can’t really remember.
Can’t have + past participle
We use ‘can’t have + past participle’ for things that we are sure did not happen in the past.
- I can’t have left my phone at work. You phoned me when I was walking to my car. That’s it. It must be in the car.
- You can’t have seen him this morning. He was with me all the time.
- She can’t have liked the show. She hates musicals.
Some exercises to practice: