Past Modals for Deduction
Use and Form:
The following modals can be used to guess what happened in the past.
must have + past participle verb
Use this when you make a guess about the past, and you are almost certain that your guess is correct.
‘The cats have escaped! How did they get out?’
‘They must have got out under the gate. There is no other way out.’
may have + past participle verb
might have + past participle verb
could have + past participle verb
Use this when you make a guess about the past, but you are only suggesting one possibility. You are not certain you are correct.
‘The cats have escaped! How did they get out?’
‘They may have got out under the gate, or they might have escaped through this hole in the fence.’
may not have + past participle verb
might not have + past participle verb
Use this when you make a guess about what didn’t happen in the past, but you are only suggesting one possibility. You are not certain you are correct.
‘Where’s Inma? Why isn’t she at the meeting?’
‘She may not have got the message.’ / ‘She might not have received the message.’
NOTE: may not is not contracted to mayn’t, and might not is rarely contracted to mightn’t.
can’t have + past participle verb
couldn’t have + past participle verb
Use this when you make a guess about what didn’t happen in the past, and you are almost certain that your guess is correct.
‘The cats have escaped! How did they get out?’
‘They can’t have got out under the gate. I fixed that last week.’
NOTE: you cannot use: mustn’t have + past participle verb to make deductions about the past.
Common Mistakes:
1. Many people do not take the opportunity to use these structures when they can.
Maybe I left my phone at home. → I may have left my phone at home.
I think Quique went to the restaurant without us. → Quique must have gone to the restaurant without us.
2. Note that could have has the same meaning as might have and may have.
Why is Inma late?
She may / might / could have got stuck in traffic.
However, in the negative form the meaning is not the same.
Why is Inma late?
She may / might not have got your message. (NOT could not)
= Maybe she didn’t get your message.
Couldn’t have has the same meaning as can’t have.
Why is Inma late?
I’m not sure! She can’t / couldn’t have forgotten about the party!
= I’m sure she didn’t forget.