Page 129 - Practical English Usage 3ed - Michael Swan, Oxford

can and could (2): ability 122 I've always been able to play games well. (NOT pfIl! aiftlays e6ukl .. .)
She's always been allowed to do what she liked.
e Could is sometimes used as the past of can.
When I was younger I could play tennis very well.
However, it can also be used as a less definite or conditional form of can, referring to the present or future (see 122.6, 124.1,4-6).
Could I ask you something? (More polite than Call I ... ?)
What shall we do this evening?- We could go and see a film.
I could get a betterjob ifI spoke French. (= I would be able to ...)
f Certain past ideas can be expressed by can or could followed by a perfect infinitive (have + past participle). For details, see 122.7, 123.5 and 124.7.
I don't know where she can have gone.
That was dangerous - he could have killed somebody.
9 Can has two pronunciations: a strong form /kam/ and a weak form /kan/. Could has a strong form /kod/ and a weak form /kad/. The weak pronunciation is used in most cases. For more details of strong and weak pronunciations, see 616.
h Contracted negative forms (see 143) are can't (pronounced /ka:nt/ in standard British English and /kamt/ in standard American English) and couldn't U'kodntj). Cannot is usually written as one word in British English.
2 meanings
Can and could are both used to talk about ability, to ask for and give permission, and to make requests and offers.
Can you speak French? (ability)
You can stop work early today. (permission) Could I have some more tea? (request)
Can I help you? (offer)
Could is also used to talk about the chances that something will happen, or is happening. Can is not used in this way.
It could rain this afternoon. (NOT It ean rain this ajteffUJtm.)
Can't is not only used to talk about ability or permission; it can also express negative certainty (see 359.2).
It can't be true. (= It is not possible that it is true.)
With see, hear and some other verbs, can is used to give a kind of present progressive meaning.
I can hear the sea. (NOT ... am hettrirtg the sea.)
May and might are often used in similar ways to can and could. For the main differences, see 345.
122 can and could (2): ability
1 knowledge. skill, strength etc: I can read Italian
We use can to say what people and things are able (or unable) to do because of their knowledge. skill, strength, nature, design etc.
I can read Italian, but I can't speak it. These roses can grow anywhere. Dogs can't climb trees. Can gases freeze?
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