Collins Cobuild

Unit 81
Possibility
Main points
You use ‘can’ to say that something is possible.
You  use  ‘could’,  ‘might’,  and  ‘may’  to  indicate  that  you  are  not
certain whether something is possible, but you think it is.
1
When you want to say that something is possible, you use ‘can’.
Cooking can be a real pleasure.
In some cases this can cause difficulty.
You use ‘cannot’ or ‘can’t’ to say that something is not possible.
This cannot be the answer.
You can’t be serious.
2
When you want to indicate that you are not certain whether something
is possible, but you think it is, you use ‘could’, ‘might’, or ‘may’. There
is  no  important  difference  in  meaning  between  these  modals,  but
‘may’ is slightly more formal.
That could be one reason.
He might come.
They may help us.

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You can also use ‘might not’ or ‘may not’ in this way.
He might not be in England at all.
They may not get a house with central heating.
Note that ‘could not’ normally refers to ability in the past.   See Unit
83
.
3
When there is a possibility that something happened in the past, but
you are not certain if it actually happened, you use ‘could have’, ‘may
have’, or ‘might have’, followed by a past participle.
It could have been tomato soup.
You may have noticed this advertisement.
You can also use ‘might not have’ or ‘may not have’ in this way.
He might not have seen me.
They may not have done it.
You  use  ‘could  not  have’  when  you  want  to  indicate  that  it  is  not
possible that something happened.
He didn’t have a boat, so he couldn’t have rowed away.
It couldn’t have been wrong.
You  also  use  ‘could  have’  to  say  that  there  was  a  possibility  of
something happening in the past, but it did not happen.
It could have been awful. (But it wasn’t awful.)
You could have got a job last year. (But you didn’t get a job.)
4
You also use ‘might have’ or ‘could have’ followed by a past participle
to  say  that  if  a  particular  thing  had  happened,  then  there  was  a
possibility of something else happening.
She said it might have been all right, if the weather had been good.
(But the weather wasn’t good, so it wasn’t all right.)
If I’d been there, I could have helped you. (But I wasn’t there, so I

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couldn’t help you.)
5
 ‘Be  able  to’,  ‘not  be  able  to’,  and  ‘be  unable  to’  are  sometimes  used
instead  of  ‘can’  and  ‘cannot’,  for  example  after  another  modal,  or
when  you  want  to  use  a  ‘to’-infinitive,  an  ‘-ing’  form,  or  a  past