can和could,should,would等词语的用法区别?

could 可以 语气较委婉 较轻
should 应该 有点上对下的语气 较强硬.
I wish you would stay.
我希望你能留下来
Used to make a polite request:
用于有礼貌的请求：
Would you go with me?
你愿意和我一起去吗?
Used to indicate uncertainty:
表示不确定之意：
It would seem to be getting warmer.See Usage Note at if
天气似乎变得更暖和了参见 if
would
[wEd, wJd]
v.
（略写为'd,否定式的略写为 wouldn't [wJd(E)nt] ）
(will 的过去式）
They said it would be fine.
听说天气会很好.
I would give anything to see the film.
假如可能的话,我怎么也要看一看这部电影.
Would you like some cake or biscuit?
你要吃点蛋糕还是并干?
He said he would come.
他说他要来.
（表示过去的习惯）常,有…的习惯
She would lose the key!
她总是把钥匙弄丢了.
would rather
宁可…也不；宁愿
Which would you rather do, go to cinema or stay at home?
你是去看电影还是待在家里?
Would rather die than surrender.
宁死不投降.
I could run faster then. It could be no better at that time. Only men could go to the club in those days.
那时我能跑得更快.那时好得不能再好了.在当时,只有男人可以去俱乐部
Used with hypothetical or conditional force:
用来表推测或条件：
If we could help, we would.
如果我们能帮上忙,我们会帮的
Used to indicate tentativeness or politeness:
用于表示试探性或礼貌：
I could be wrong. Could you come over here?
我也许错了.你能到这边来吗?
could
[kJd]
v.
动词 can 的过去式
I could run faster then.
我那时能跑得更快一些.
（表示与事实相反的设想）能；可以
I could come tomorrow if you like.
如果你愿意,明天我可以来.
（委婉语）能
Could you do sth. for me?
你能替我做件事情吗?
应该
You could at least have met me at the station.
你至少应该到车站来接我.
可能
I wrote down the number so that I could remember it.
我写下了号码,那样我就能记住它了.
You should send her a note.
你应该给她留一个条
Used to express probability or expectation:
用于表示可能性或期望：
They should arrive at noon.
她应该在中午到达
Used to express conditionality or contingency:
用于表示可能性或可能发生的事件：
If she should fall, then so would I.
如果她失败了,那我也会失败
Used to moderate the directness or bluntness of a statement:
用于使一个直接或直率的陈述变得婉转：
I should think he would like to go.
我倒是认为他愿意去
Like the rules governing the use of shall and will on which they are based, the traditional rules governing the use of should and would are largely ignored in modern American practice. Either should or would can now be used in the first person to express conditional futurity:
就象作为shall 和 will 词的基础的限用的用法规则一样, 适用于should 和 would 这个词的传统使用规则在现代美国英语中也已被忽略了. 现在should 或 would 这两个词中的任何一个都可以用于第一人称,表示条件式中的将来：
If I had known that, I would (or somewhat more formally, should ) have answered differently.
如果已经知道了这个情况的话,我就（或正规一点用 should ） 不会那么回答了 ,
But in the second and third persons only would is used:
但在第二人称或第三人称中只用would ：
If he had known that, he would (not should ) have answered differently.
如果他知道那个情况的话,就（不能用 should ） 不会那么回答了.
Wouldcannot always be substituted for should, however. Should is used in all three persons in a conditional clause:
但是Would并不是总是能由 should 代替. Should 在三种人称的条件从句中都可以用：
if I (or you or he ) should decide to go.
如果我（或 你 或 他 ） 决定要去.
Should is also used in all three persons to express duty or obligation (the equivalent of ought to ):
Should用于这三种人称的表示职责和义务的句子中（相当于 ought to ）：
I (or you or he ) should go.
我（或者 你 或者 他 ） 应该去 .
On the other hand, would is used to express volition or promise:
另一方面,would 用来表达决心或保证：
I agreed that I would do it.
我一定会做的.
Either would or should is possible as an auxiliary with like, be inclined, be glad, prefer, and related verbs:
而would 或 should 都可以作助词和 like,be inclined, be glad,prefer 及其相关词语一起使用：
I would (or should ) like to call your attention to an oversight.
我想（或 should ） 请你注意一下一个疏漏之处 .
Here would was acceptable on all levels to a large majority of the Usage Panel in an earlier survey and is more common in American usage than should. · Should have is sometimes incorrectly written should of by writers who have mistaken the source of the spoken contraction should've. See Usage Note at if, rather, shall
在此处,在一次早期的调查中对大多数各阶层的使用者来说,would 是可以接受的, 且在美国用法中比should 更为常见. 有时书写者把should have 误拼成 should of , 因为他们把口头缩略形式should've的来源给弄错了 参见 if, rather, shall
should
[Fed, FJd]
v.aux.
否定式缩略为 shouldn't
（用于间接引语,表示）将
We said we shouldn't arrive till 6.
我们说过我们六点才能到.
"If it should rain tomorrow, the sports meeting would be postponed."
"要是明天下雨,比赛就延期举行."
（和that一起用于表示意愿的动词和形容词之后）
He was keen that she should go to college.
他渴望她能上大学.
（在条件句中与第一人称连用） 可能；会
I should have bought it if I had enough money.
如果当时我有足够的钱,我会把它买下来的.
（表示责任或义务）必须,应该
Children should obey their parents.
儿童应该服从他们的父母.
He should have known that the police would never allow this sort of thing.
他本应知道这样的事情警察是决不允许的.
（表示可能之事）可 能
It should be fine tomorrow.
明天可能天晴.
（表示不确定）万一
If I should see him, I'll tell him.
万一我见到他,我就告诉他.