英语四级语法基础知识：

Ⅱ 非谓语动词

不定式

形式：主动形式、被动形式

一般式 to do to be done

完成式 to have done to have been done

进行式 to be doing

完成进行式 to have been doing

完成式：不定式的一般形式所表示的动作，通常与主要谓语表示的动作(状态)同时(或几乎同时)发生，或是在它之后发生。假如不定式所表示的动作，在谓语所表示的动作(状态)之前发生，就要用不定式的完成式。

I am glad to have seen your mother (= I am glad I have seen your mother).

(比较: I am glad to see you.)

He is said to have written a new book about workers.

He pretended not to have seen me.

进行式：如果主要谓语表示的动作(状态)发生时，不定式表示的动作正在进行，这时要用不定式的进行式。

You are not supposed to be working. You haven't quite recovered yet.

We didn't expect you to be waiting for us here.

He pretended to be listening attentively.

完成进行式：在谓语所表示的时间之前一直进行的动作，就要用不定式的完成进行式。

The struggle was known to have been going for twenty years.

We are happy to have been working with you.

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