We could not have heard them because of the noise from the river. 由于有河水声我们不能听见他们的话。
Can he have left already? 他会已经离开了吗?
They can’t have gone out because the light’s on. 他们不可能出去了，因为灯还亮着。
注意：在非正式英语或口语中，偶见到could have done也可用于肯定句，表示推测：
She could have forgotten to tell him. 她可能忘记告诉他了。
Tom could have taken the money; he was here alone yesterday. 有可能是汤姆把钱拿走了，昨天他单独一个人在这里呆过。
In the example mentioned above, either method of construction could have been used.在上面提到的例子中，两种建造方法均可能用过。
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译文：其他皇室成员也被允许佩戴龙的象征，但爪子更少，颜色也不同。
重点：“of＋名词结构”
名词是use, importance, help, value, interest, benefit等抽象名词时，
该结构相当于“be＋名词所对应的形容词”，这类名词前可用no, some, any, little, much, great等对其进行修饰。
名词是size, kind, type, price, height, depth, width, length, weight, age, shape, colour等时，该结构表示不同的人或物具有某种共同特征，这类名词前可用a, an, the same, different等修饰。
There have been reports of monsters in Lake Tianchi since the beginning of the last century, although no one has seen one close up.
译文：自上世纪初以来，就有关于天池怪物的报道，尽管还没有人近距离看到过。  
重点：there be结构的完成时（There+have been） 

could have done 表示未曾实现的可能性
该结构仅限于使用could，不用can。表示过去本来有能力、有可能、有打算、有选择做某事的，但实际上却没有那样做。即“过去本来可能做某事，而实际没有做”。通常译为“本来可以”，“本来能够”等。例如：
You could have done better, but you didn't try your best.你本可能做得更好，但你没有尽力。
We could have got in for nothing—nobody was collecting tickets. 我们本可以不买票就进去——根本没人收票。
I could have passed my examination easily but I made too many stupid mistakes. 我本可以轻易通过考试，但我犯了太多不该犯的错误。
You could have stayed with me when you were in this city on business.在这个城市出差时你本能够和我住在一起的。
She could have been seriously injured. 她本来可能伤得很重的。
It’s difficult enough, but it could have been worse. 这事情够困难的了，本来可能会更糟呢！
I could have given you some hints, but I suppose you thought yourself too grand. 我本可以给你点提示的，但是我认为你太自负了。

一般规律
在通常情况下，英语比较级前不用冠词，形容词最高级前用定冠词，副词最高级前可以用定冠词，也可以不用。如：
This homework is all right but you could do it better. 这份家庭作业还算可以，其实你可以做得更好。
That is the best way to prevent such a thing happening again. 这是防止这样的事再次发生的最好办法。
The boss praised those who had worked (the) hardest. 老板表扬了那些工作最努力的人。
比较级前冠词使用规律
比较级前通常不用冠词，但有时也用，这主要见于以下情形：
当比较级后接名词时，比较级前可能用冠词，此时的冠词不是修饰比较级，而是修饰其后的名词。如：
Which of them is the better choice?—Well, it’s hard to tell. 他们谁是更合适的人选?——嗯，这很难说。
Some of the more time-consuming jobs can now be done by machines. 有些更费时间的工作现在可以用机器做了.
但是，若受比较级修饰的名词为表泛指的复数名词或不可数名词，则仍不用冠词。如：
He’s got more money than the rest of us (put) together. 他的钱比我们大家的加在一起的总数还多。
Fewer people write with their left hand than with their right. 用左手写字的人比用右手的少。
当要特指两者中“较(更)……”时，比较级前通常要带定冠词，此时可视为比较级后省略了有关的名词或代词one，即定冠词实为修饰被省略的名词或代词one。如：

You ________ be Carol. You haven't changed a bit after all these years.
A. must
B. can
C. will
D. shall
You ________ feel all the training a waste of time, but I'm a hundred percent sure later you'll be grateful you did it.
A. should
B. need
C. shall
D. may

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