could与 can有什么区别?

could与 can有什么区别?
could与 can有什么区别?

could与 can有什么区别?
Sian Harris answers:
　　This is a common area for questions,as these verbs can have several
functions.Without going into too much detail,I'm going to try and illustrate
the key uses for you.
　　Firstly,we use 'can' for something that is possible or to show that
somebody has the ability to do something in the present and future.For
example:
　　We can see the park from our house,
　　Or
　　Ella can speak fluent Japanese.
　　The negative here is 'can't'...as in:He can't swim very well.
　　In this case,'could' is generally used as the past form of 'can'.So,
　　Ella could speak fluent Japanese when she was young,and she can speak
several other languages now too.
　　Or:
　　When I was a child I could run fast.
　　We use 'could' for general past ability,but be a little bit careful here,
because when we're talking about what happened in a particular situation,we
tend to use 'was/were able to' for past ability instead:
　　For example:The fire spread quickly,but luckily everybody was able to
escape.
　　We also use 'could' to talk about possible actions now and in the future -
here the function is possibility (not ability) - and this is what tends to cause
the confusion around these words.For example,if you are expecting some friends
to visit,but they have been delayed,you might say 'they could arrive at any
time now',or if you're trying to make progress with your work you might ask
'Could we talk to the boss again?For this function of possibility,we need to
watch out for the past form,as this doesn't work in exactly the same way.If we
want to express past possibility,we need to use could + have and the past
participle (have done,have been etc).Let's imagine you've received a letter or
card from a mystery admirer,and you're trying to work out who sent it to you.
As the sending happened in the past,we are speculating about a past
possibility.'John could have sent it' but perhaps 'James could have written
it'...we are not sure who,but think there are some possibilities.
　　Finally,let's look at how we commonly use can/could in question forms to
make requests (or ask for things).If we go shopping for clothes,we might ask
'can/could I try that dress on please?' or 'could I see those shoes in blue.'
Here,either 'can' or 'could' may be used without significant difference.The
only thing worth noting is that in terms of register,that is (style and
formality),'could' is considered slightly more formal or polite.
　　So,to recap,three of the main functions for 'can/could' are to talk
about:ability,possibility (with the change in the past form to remember) and
for requests.
　　Sian Harris is the Manager of English Language Training & Development
at the BBC World Service,and runs specialist courses in London and overseas for
BBC staff.Before joining the BBC,she spent 10 years as an English language
teacher,examiner and academic manager in schools and colleges in London.
1、can 表示现在的能力,could 表示过去的能力,例如：He can speak a little English now.
　　She could play the piano when she was five.
　　2、在表示请求许可时,没有时间区别,在语气上could 更加委婉客气,例如：
　　Can I use your pen
　　Could I have a look at your book
　　3、在表示可能性方面,没有时间区别,can 可能性比could 大;1、can 表示现在的能力,could 表示过去的能力,例如：He can
speak a little English now.
　　She could play the piano when she was five.
　　2、在表示请求许可时,没有时间区别,在语气上could 更加委婉客气,例如：
　　Can I use your pen
　　Could I have a look at your book
　　3、在表示可能性方面,没有时间区别,can 可能性比could 大;表示逻辑上或理论上的可能性,而不是某种实际上将要发生或正在发生的可能性.
　　Can it be true?这会是真的吗?
　　Anybody can make mistake.任何人都可能犯错误.
　　The weather here could be very cold in winter.冬天这的天气有时很冷.
　　一般用于否定句和疑问句.
　　He can't be at home.他不可能在家.
　　注意和may not 的区别.
　　He may not be at home.他可能不在家.
　　也可以用于肯定句.,
　　An accident can happen if you do this.
　　如果你这样做,可能会发生事故

1、can 表示现在的能力，could 表示过去的能力，例如：He can speak a little English now.
She could play the piano when she was five.
2、在表示请求许可时，没有时间区别，在语气上could 更加委婉客气，例如：
Can I use your pen ?
Could I ha...

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1、can 表示现在的能力，could 表示过去的能力，例如：He can speak a little English now.
She could play the piano when she was five.
2、在表示请求许可时，没有时间区别，在语气上could 更加委婉客气，例如：
Can I use your pen ?
Could I have a look at your book ?
3、在表示可能性方面，没有时间区别，can 可能性比could 大；1、can 表示现在的能力，could 表示过去的能力，例如：He can speak a little English now.
She could play the piano when she was five.
2、在表示请求许可时，没有时间区别，在语气上could 更加委婉客气，例如：
Can I use your pen ?
Could I have a look at your book ?
3、在表示可能性方面，没有时间区别，can 可能性比could 大；表示逻辑上或理论上的可能性，而不是某种实际上将要发生或正在发生的可能性。
Can it be true? 这会是真的吗？
Anybody can make mistake.任何人都可能犯错误。
The weather here could be very cold in winter. 冬天这的天气有时很冷。
一般用于否定句和疑问句。
He can't be at home.他不可能在家。
注意和may not 的区别。
He may not be at home. 他可能不在家。
也可以用于肯定句。，
An accident can happen if you do this.
如果你这样做，可能会发生事故

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could是“can”的过去式;