When we say that someone or something has or doesn't have the ability to do something, we can use can('t) (or cannot) (for the present) or could(n't) (for the past):
• He can analyse people's handwriting. • We can't afford to pay the bill.
• Anita could speak three languages before she was six.
We can use be able to instead of can/could to talk about an ability that someone has or had:
• Helen can / is able to read well, even though she's only three.
• He could / was able to draw on the support of over 20,000 troops.
In general, we use be able to when we talk about a specific achievement (particularly if it is difficult, requiring some effort) rather than a general ability.
However, we commonly use can or could, even when we are talking about specific occasions, with verbs of the senses, feel, hear, see, smell, taste, and with verbs of 'thinking', e.g. believe, decide, remember, understand:
• She could feel the spray on her face as the boat raced through the water.
• I can't decide where to go for my holidays.
We don't use be able to
• when we talk about something that is happening as we are speaking:
• before be + past participle:
• This furniture can be assembled by anyone, with just a screwdriver.
We use be able to, not can or could, if the idea we want to express needs a to-infinitive, an -ing form, or a perfect tense, or if it follows another modal verb:
• We were very lucky to be able to live in the country during our childhood.
• It was awful, not being able to see you for so long.
• Since he left, none of the other team members have been able to match his enthusiasm.
• We may be able to move some of the staff to a different department.
We use will be able to, not can, to say that something will be possible in the future:
• If the snow carries on like this, very few people will be able to get to the concert.
• When the new road is built, I'll be able to drive to work in under half an hour.
However, when we make a decision now about something in the future, we use can:
• You can go home when you've finished writing your composition.
• Perhaps we can meet next week.
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