英語四級語法基礎知識：

Ⅱ 非謂語動詞

不定式

形式：主動形式、被動形式

一般式 to do to be done

完成式 to have done to have been done

進行式 to be doing

完成進行式 to have been doing

完成式：不定式的一般形式所表示的動作，通常與主要謂語表示的動作(狀態(tài))同時(或幾乎同時)發(fā)生，或是在它之后發(fā)生。假如不定式所表示的動作，在謂語所表示的動作(狀態(tài))之前發(fā)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).

(比較: I am glad to see you.)

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

He pretended not to have seen me.

進行式：如果主要謂語表示的動作(狀態(tài))發(fā)生時，不定式表示的動作正在進行，這時要用不定式的進行式。

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.

完成進行式：在謂語所表示的時間之前一直進行的動作，就要用不定式的完成進行式。

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

We are happy to have been working with you.

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