来自：查字典高考网 2017-03-04
Can和Could 都是情态动词。一般情况下，Could比Can 更礼貌（或者更正式）一些。下面分别来介绍 Can和Could的一些用法。
CAN
The three main uses of Can are: 1) ability 2) possibility and 3) permission. However there are some other uses of Can as we will see below.
1. To express ability
Can means to be (physically) able to do something OR to know how to do something.
I can speak two languages.
2. To express a possibility (in general)
This refers to a theoretical possibility.
Note: we don't use can to talk about future possibilities. For this you would use may or might.
I know you can win the competition.
3. To offer to do something for others
Can is used when you offer to help someone or to do something for them.
You don't need to walk home. I can take you there if you like.
4. To ask for or give permission / To request something
Can is used to ask for / request permission or to give permission.
Note: Can't is used to refuse permission.
Can means to be (physically) able to do something OR to know how to do something.
Mary, you can play outside if you like.
5. Can't: Something that is forbidden or not allowed
You can drive as fast as you want on the German motorways.
COULD
Could is similar to Can and often replaces Can in the past tense (though not always)
1. To express ability in the past
She could juggle eight balls when she was only 10 years old.
2. Polite word used to ask for permission or to request something (in the present)
Could I please use your bathroom?
3. General permission in the past
He couldn't go to the concert because his mother wouldn't let him.
4. Couldn't: Sure that something is untrue
Just like can't, you can use couldn't when you are sure that something isn't true or real.
He couldn't have painted that. He has no artistic ability at all.
5.Could: A conditional form ofCan
Would + Can = Could. Note: Would be able to can replace could in the following examples.
I could write the letter if you told me what to include.
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