Past Modals

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, andyou are almost certain that your guess is correct.
‘The chickens have escaped! How did they get out?’
‘They must have got out under the gate. There is no other way out.’
may have + past participleverb
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 arecorrect.
‘The chickens 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
mightnot 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 John?Why isn’t he at the meeting?’
‘He may not have got the message.’ / ‘He might not have received the message.’
NOTE: may not is not contracted to mayn’t, and might not is rarely contracted tomightn’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 chickens have escaped! How did they get out?’
‘They can’t have got out under the gate. I fixed that yesterday.’
NOTE: you cannot use: mustn’t have + past participle verb to make deductions aboutthe past.
Common Mistakes:
1. Many students do not take the opportunity to use these structures when they can.
Maybe I left my book at home. → I may have left my book at home.
I think Robin went tothe restaurant without us. → Robin 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 Tom late? 
He may / might /...