modal verb
1
DUBIOUS : Can you possibly send me an application form?
GOOD : Could you possibly send me an application form?
DUBIOUS : I'd be grateful if you can confirm whether you are coming.
GOOD : I'd be grateful if you could confirm whether you are coming.
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To make a polite request, use could : 'Could you pass the butter, please?'
2
BAD : The next generation can fly to the moon for their holidays.
GOOD : The next generation will be able to fly to the moon for their holidays.
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Use can to talk about someone's present ability: 'All our children can swim.'
Use be able to to predict someone's future ability: 'You'll never be able to swim if you don't try.'
3
See REMEMBER (↑ remember )
4
See COULD (↑ could )