oxford_guide_to_english_grammar

12 MODAL VERBS
PAGE 124
3 Possibility in the past
May/might/could + perfect refers to something in the past that is possibly true.
Miranda may have missed the train.
(= Perhaps Miranda missed the train.)
The train might have been delayed.
(= Perhaps the train has been delayed.)
The letter could have got lost in the post.
(= It is possible that the letter has got lost in the post.)
NOTE
Could have done can also mean that a chance to do something was not taken. • 98(3d)
I could have complained, but I decided not to.
98 Ability: can, could and be able to
1 Can and could
We use these verbs to say that something is possible because someone has the
ability to do it. We use can for the present and could for the past.
Nicola can play chess.
Can you draw a perfect circle?
We can't move this piano. It's too heavy.
Nicola could play chess when she was six.
My grandfather could walk on his hands.
The negative of can is cannot , written as one word. It has a short form
can't
As well as physical or mental ability, we also use can/could for a chance, an
opportunity to do something.
We can sit in the garden when it's nice.
When we lived in aflat, we couldn't keep a dog.
NOTE
a With some verbs we can use a simple tense for ability.
I (can) speak French. We didn't/couldn't understand the instructions.
b For can/could expressing a perception, e.g. I can see a light, • 62(7).
2 Be able to
a
b
c
Be able to in the present tense is a little more formal and less usual than can.
The pupils can already read/are already able to read.
The duchess can fly/is able to fly an aeroplane.
We use be able to (not can) in the perfect and the infinitive or ing-form.
Mr Fry has been ill for years. He hasn't been able to work for some time.
It's nice to be able to relax.
Being able to speak the language is a great advantage.
We use will be able to for future ability or opportunity.
When you have completed the course, you will be able to impress others with your
sparkling conversation.
One day people will be able to go on a package tour of the solar system.