Used in the negative, be to can also express prohibitions:
You
are not to do
that again!
Can, could and may are used to ask for permission. Can is the most direct of the three, could is more formal and tentative, and may is the most formaclass="underline"
Can
I
use
your phone?
Could
I
have
a glass of water?
May
I
start
my presentation?
Might, which has a more tentative meaning, is very formal and is rarely used:
Might
I
ask
you a question?
When we talk about permission but we are not actually giving, refusing or asking for it, we can use can/can't or be allowed to:
Students
can choose
any topic for their project.
We
can't eat
in the library.
Are
we
allowed to smoke
in this restaurant?
Will
journalists
be allowed to ask
questions?
Past
Could can be used to express general permission in the past:
When I was a child, I
could do
basically anything that I wanted to do.
But when we talk about a particular action which was permitted and performed, we use was/were allowed to:
Although I was underage, I
was allowed to enter
the race.
Modals to express possibility: MAY, MIGHT, CAN, COULD
Present and future
May and might + infinitive are used to express present or future possibility. May expresses a greater degree of certainty:
You should ask him. He
may/might know
Susan's telephone number.
(Perhaps he knows her number.)
I
may/might see
you later.
(Perhaps I will see you later.)
You should introduce yourself; he
may/might not remember
you.
(Perhaps she doesn't/won't remember you.)
May and might are usually not used to introduce a question. Instead, we can use Do you think? or be likely to / be likely that:
Do you think
he may/might know Susan's telephone number?
Are
you
likely to get
here before 8?
Is it likely that
you will get here before 8?
Could can be used instead of may and might with the verb be:
You
could be
right.
They
could
still
be waiting
for us.
The negative form couldn't is often used with comparative adjectives:
The food is delicious, and the staff
couldn't be
more polite.
(they are very polite)
Except for this use, couldn't expresses negative deduction, not possibility:
It's only 10 o'clock. He
couldn't be
at home.
(He is usually at work at this time of the day.)
Can may express general possibility:
Winters in Minnesota
can be
really cold.
Past
May, might and could + perfect infinitive express uncertainty with reference to past actions:
We haven't heard from him for ten years. He
may/might/could have died
.
(Perhaps he has died, but we don't know.)
But when we want to say that something was possible but did not happen, we use might or could:
He was very careless when crossing the road. He
might/could have died
.
(He didn't die.)
I
could have caught
the bus if I had hurried.
(I didn't hurry, so I didn't catch the bus.)
Couldn't + perfect infinitive is often used with comparative adjectives:
It was a great year, and I
couldn't have been
happier.
(I was very happy)
May/might not + perfect infinitive is used for uncertainty, but could not + perfect infinitive (except for the case above) expresses deduction:
I had better call Anne. She
may/might not have read
my e-mail.
(uncertainty)
It
couldn't have been
John you saw this morning. He is away on holiday.
(deduction)
Quotes:
It is never too late to be who you might have been. - George Eliot
If you don't know where you're going, you might never get there. - Yogi Berra
Related topics:
Modals to express deduction: MUST, CAN'T, COULDN'T
Modals to express predictions: BE GOING TO, WILL