adjective
1
BAD : One man is able to destroy the whole world.
GOOD : One man is capable of destroying the whole world.
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If someone is able to do something, they can do it and it is not unusual or surprising if they do it: 'The doctor said that after a few days I'd be able to get out of bed.' 'Will you be able to play on Saturday?'
If someone is capable of (doing) something, they do not usually do it, but it is possible for them to do it if they want to: 'I'm sure he's quite capable of getting here on time, but he can't be bothered.' 'The power station is capable of generating enough electricity for the whole region.'
2
BAD : There are so many places to visit in London that I'm not able to decide where to go.
GOOD : There are so many places to visit in London that I can't decide where to go.
BAD : We weren't able to stop laughing.
GOOD : We couldn't stop laughing.
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See also COULD 1 (↑ could )
3
BAD : In some countries you are not able to drink until you are 21.
GOOD : In some countries you can't drink until you are 21.
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For actions that are controlled by laws or rules, use can, can't, etc, or be (not) allowed to : 'Now that they are both sixteen, they can get married.' 'The goalkeeper can touch the ball, but nobody else can.'
4
BAD : Technology has made them able to grow their own food.
GOOD : Technology has enabled them to grow their own food.
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enable = make someone able to do something: 'This scheme is designed to enable young people to find work.'