We use can to say that something is possible or that somebody
has the ability to do something.
We use can + infinitive (can
do / can see etc.) :
- We can see the lake from our bedroom window.
- Can you speak any foreign languages ?
- I can come and see you tomorrow if you like.
The negative is can't (= cannot):
- I'm afraid I can't come to the party on Friday.
(Be) able to... is posible instead of can, but can is more usual:
- Are you able to speak any foreign languages ?
But can has only two forms, can (present) and could (past). So sometimes
it is necessary to use (be) able to... .
Compare:
but
- I haven't been able to sleep recently. ( can has no present perfect)
but
- Tom might be able to come tomorrow. (can has no infinitive)
Could and was able to...
Sometimes could is the past of can. We use could especially with:
see , hear , smell , taste , feel , remember , understand
- When we went into the house, we could smell burning.
- She spoke in a very low voice, but I could understand what she said.
We also use could to say that somebody had the general ability or permission
to do something
- My grandfather could speak five languages.
- We were completely free. We could do what we wanted. (= we were allowed
to do...)
We use could for general ability. But if we are talking about what
happened in a particular situation, we use was
/ were able to... or managed
to... ( not could):
- The fire spread through the building quickly but everybody
was able to escape.
or ...everybody managed to escape. (but not 'could escape')
- They didn't want to come with us at first but we managed
to persuade them.
or ...we were able to persuade them. (but not 'could persuade')
Compare:
- Jack was an excellent tennis player. He
could beat anybody.
(= he had the general ability to beat anybody)
but
- Jack and Alf had agame of tennis yesterday. Alf played very
well but in the end Jack managed to beat him. or ...was
able to beat him. (= he managed to beat him in this particular game)
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The negative couldn't ( could not ) is possible in all situations:
- My grandfather couldn't swim.
- We tried hard but we couldn't persuade them to come with us.
- Alf played well but he couldn't beat Jack.
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