1. may和might+ 动词完成式

此结构用于推测过去的行为。例如：

She said that he might have missed the plane.她说他可能误了航班。

2. can和could+ 动词完成式

A. 表示过去能做而没做的事，有一种对过去未付诸实施的事情的惋惜。例如：

In those circumstances we could have done better.

在那样的情况下，我们本来可以做得更好的。(但没有做得更好)

B. 推测过去的某种行动。

Where can /could she have gone? 她可能到哪里去了呢?

The boy can’t have finished reading the book so soon because it is difficult even to an adult.

这孩子不可能那么快就把这本书读完的，因为即使对一个成人来说，这本书也是相当难的。

Note:

can’t和couldn’t+ 动词原形可以用来表示否定推断。例如：

He can’t be over fifty.他不可能超过50岁。

3.must +动词完成式

表示对过去行为的推断，具有较大的可能性，意为“一定…，想必…”。例如：

She must have gone through a lot.

她一定吃过很多苦。

Note:

must+动词原形→对现在的推测，must be+现在分词→对未来或现在正在进行的推测。例如：

She looks happy; she must be having a good time.

4. needn’t+动词完成式和didn’t need to do

A. needn’t+动词完成式表示一种已经做过的但并无必要的行为。例如：

You needn’t have watered the flowers, for it is going to rain.

你本来不必浇花的，因为天要下雨了。(但却浇了)

We needn’t have told him the news because he knew it already.

B. didn’t need (have) to do结构表示没有必要做某事，实际上也没有做。例如：

I didn’t need to clean the windows. My sister did it.

我不必擦窗，我妹妹擦了。(我没有擦)

5. should +动词完成式

A.此结构的肯定式表示应该做的事而没有做，否定式表示某种行为不该发生但却发生了。例如：

You should have given her more help.

你应该多给她一些帮助的。(但没有给)

B. 表示推测，意为“可能”，但可能性较小。例如：

He should have finished the work by now.

他现在该把工作完成了。(推测)

C. should / would have thought意为“本以为，本认为”。例如：

A: "Can you type?"

B: "Certainly."

A: "Well, I should have thought you wouldn’t."

I should have thought she wouldn’t agree.

我本来认为她不会同意的。

D. should have thought有时意为“认为，以为”，相当于should think，但表示更为委婉、谦逊或不肯定的语气。例如：

I should have thought it fairly good.

我以为它是很不错的。

E. should have + 过去分词用于虚拟语气：

Had you written him, you should have known the details.

6. ought to + 动词完成式

此结构表示某种过去应该做而没有做的事，或被忽略未做的某种明智行为。例如：

You ought to have returned the book earlier.(还晚了)

You ought to have refused her at the beginning, but now it is too late.

Note:

这里的ought to也可以用should代替。

Homework:用给出的动词填空,注意要选用正确的助动词和时态.

1.The boy____ the book so soon because it is difficult even to an adult. (read)

2.We___him the news because he knew it already. (tell)

3.If we had taken the other road, we ____earlier. (arrive)

4.I _____you might take this into consideration. (think)

5.You ____ her at the beginning, but now it is too late.(refuse)