We use can to describe an ability and cannot or can't to describe a lack of ability in the present :
- She can speak Spanish but she can't speak Italian.
If the present ability is surprising or involves overcoming some difficulty, we can also use is/are able to:
-Despite his handicap he is able to drive a car.
To emphasise the difficulty or to suggest a great effort (in the present, past or future) we use manage to. To describe a future ability we use will be able to, not can :
X Gan l speak fluently by the end of the course?
• Will I be able to speak fluently by the end of the course?
We also use be able to where can/could is grammatically impossible, for example:
I haven't been able to drive since I dislocated my wrist (with the perfect aspect)
We love being able to talk the local language (with -ing forms and infinitives)
Another form for not be able to is be unable to. It is more common in formal English:
The lawyer was unable to persuade the jury of her client's innocence. We use can (present) and could (past) before sense verbs:
I think I can smell something burning in the kitchen.
We use could to describe the possession of an ability in the past:
Mozart could play the piano at the age of five.
То describe the successful use of an ability on a specific occasion we do not use could, we use a past tense or was/were able to:
Mike's car broke down but fortunately he was able to repair it. But we can use could in questions, and in sentences with limiting adverbs such as only or hardly:
Could you fix the computer yourself?' 'No, I could only back up the key files.'
She was so exhausted she could hardly speak.
We use couldn't or was/were not able to to describe a lack of ability or success:
Mozart couldn't speak French,
Despite being a mechanic, Mike couldn't fix his car when it broke down yesterday.