Grammar — Past modals of speculation
- For a past possibility we use could/might have + past participle:
- They could/might have been to London. (but I’m not sure)
- For a negative deduction about the past, we use can’t/couldn’t have + past participle:
- He can’t have left, his car’s here.
- She couldn’t have revised because she failed her exams.
- When we are positive about the past, we use must have + past participle:
- It must have been in January 1953 when grandpa went to Paris.
- When we are less certain, we use may/might not have + past participle:
- You’d better take your laptop because Ali may/might not have taken his.
- We use would have + past participle when we are making an assumption or drawing a logical conclusion:
- He would have been well-off at that time. (assumption)
- Heba would have graduated from school so she got a well-paid job. (logical conclusion)