can用于肯定句中表推测的用法：

—Do you believe what he says?

—Yes,it __________ be true.

A.can B.could C.need D.has to

此题应选B.该题容易误选A.下面谈谈can,could表示推测的有关用法：

1.表示理论上的可能性(即从理论上分析是可能的,但实际未必会发生),可用 can(表现在)或 could(表过去)：

He is in poor health.He can be ill at any time.他身体不好,随时都可能会生病.

Even experts can make mistakes.即使是专家也可能会出差错.My mother could be very unpleasant at times.我母亲有时候会让人非常不愉快.

2.表示现实可能性,这包含两个方面的含义：一是指将来可能性,一是指现在的可能性,在这两种用法中,通常都不用 can,但可用 could(可以指现在,相当于 may / might)：

今年夏天我们可能要去日本.(将来可能性)

正：We could [may,might] go to Japan this summer.

误：We can go to Japan this summer.

你可能是对的,但我不认为你对.(现在可能性)

正：You could [may,might] be right,but I don’t think you are.

误：You can be right,but I don’t think you are.

有时也用can来表示现在的可能性,但那通常只限于否定句或疑问句中：

It can’t be true.那不可能是真的.

What can he possibly want?他可能会想要什么呢?

can have done用法小结：

can have done

通常只用于否定句和疑问句。用于否定句时表否定推断，推测肯定没做；用于疑问句时意为“难道真的做了吗”，表示怀疑。例如：

Can he have done such a foolish thing? (= Is it possible …?)

他会做这样的傻事吗？（表怀疑）

He can't have taken it home.

他不可能把它带回家了。（表不可能）

could have done

could have done 可用于肯定句、疑问句和否定句。用于肯定句既可以表示过去能够做而实际未做，有惋惜、遗憾或委婉批评之意，也可以表示对过去动作的推测；用于否定句和疑问句的情况与can have done相同。例如：

You could have done better, but you didn't try your best.

你本可以做得更好，但是你没有尽力而为。（表惋惜）

This glass is cracked. Someone could have dropped it.

这个玻璃杯破了。可能是谁把它掉在地上了。（表推测过去）

Could he have left the work unfinished?

他会丢下工作不干了？（表怀疑）

He couldn't have checked out so early.

他不可能这么早就离开了。（表不可能）

注意：can't have done 的语气比 couldn't have done 要强一些。例如：

He can't have been to that town.

他根本不可能去过那座城市。（语气略强）

The measurement couldn't have been wrong.

测量不可能出错了吧。（语气稍弱）

may have done用法小结：

may/might have done 推测过去

“may/might+动词的完成式”表示对过去发生的行为的推测。例如：

-Whatever has happened to George?

-乔治发生了什么事?

-I don't know. He may have got lost.

-我不知道，他可能迷路了。

You might have read about it in the papers.

你可能在报上已经看过这个消息了。

（之前我们做过can have done用法小结，can have done表推测时都是对过去进行推测，但是may have done不仅可以推测过去）

may/might have done 推测现在

“may/might+动词的完成式”表示对现在情况的推测。例如：

She might have got up now.

现在她可能已经起床了。

may/might have done 预测未来

“may/might+动词的完成式”表示推测将来某时之前的情况。例如：

He may have left when you get there.

等你到达的时候他可能已经离开了。

注意：may have done 和 might have done 表推测时，might 所表示的可能性比 may 小，语气较委婉：

How quickly time passed! It might have happened yesterday.

时间过得好快啊！这事好像就发生在昨天。

He might have seen the surprise in my face.

他或许已经看到了我惊异的表情。

might have done 表轻微责备

might have done 除了能够表推测，还能表示过去本来可以做而实际并没有做。这时带有轻微的责备之意。例如：

The proposal might have been refused.

这个建议本该拒绝的。（有轻微的责备之意）

I might have taken another path.

我本可以走另一条路。

may not/mightn't have done 表“可能不”

之前我们讲到过 can't/couldn't have done 是表示“不可能”，而 may not/mightn't have done 则是表示“可能不”，要注意区分哦：

He can't have been at the meeting, he's on a business trip this week.

他不可能有出席会议，他这周出公差。

We couldn't have done it without you.

没有你我们肯定完成不了。

They may not have known it beforehand.

他们事先可能不知道这事儿。

They mightn't have regarded me as their friend.

他们可能没有把我当作朋友看待。