一、基本用法

當(dāng)不定式、動(dòng)名詞、從句等用作主語(yǔ)時(shí)，避免頭重腳輕，通常在句首使用形式主語(yǔ)it，而把真正的主語(yǔ)放在句子末尾：

Is it necessary to tell his father everything? 有必要把這一切都告訴他父親嗎?

It’s no good sitting up too late. 熬夜沒(méi)有好處。

It is quite clear that he has read the book. 很顯然，他讀過(guò)這本書(shū)。

It doesn’t matter what he says. 他說(shuō)什么沒(méi)關(guān)系。

二、幾種特殊的形式主語(yǔ)

(1) 用作動(dòng)詞look, seem, appear, happen, occur, follow等的主語(yǔ)(后接that從句或as if從句)：

It seems as if it is going to rain. 看來(lái)要下雨了。

It seems that he knows everything. 他好像什么都知道。

It (so) happened that I had no money with me. 碰巧我沒(méi)帶錢(qián)。

From this witness it follows that he must be guilty. 根據(jù)該證人的證詞判斷，他一定有罪。

(2) 當(dāng)系表結(jié)構(gòu)后接有if 或when引出的狀語(yǔ)從句時(shí)，通常在系表結(jié)構(gòu)前使用形式主語(yǔ)表示說(shuō)話人對(duì)某情況的看法：

It was a great surprise to me when he did a thing like that. 他這樣做使我很吃驚。

It would be a tragedy if anything went wrong. 如果出了什么事就是一個(gè)悲劇。

(3) 用于其他固定句型，如if it were not for…(若不是因?yàn)?，it’s time that…(該……了)等。

三、誤用形式主語(yǔ)的幾種情形

(1) 在該用it作形式主語(yǔ)的地方而誤用其他詞：

Is ______ necessary to tell his father everything?

A. it B. that

C. what D. he

此題的正確答案應(yīng)是A(it用作形式主語(yǔ))，而不能選D。

(2) 在不該用形式主語(yǔ)的地方而誤用形式主語(yǔ)：

______ must be something wrong with the machine.

A. There B. It

C. He D. That

此題應(yīng)選A，為there be句型與情態(tài)動(dòng)詞must連用的情形。

比較以下兩組句子：

______ is known to us all, the earth turns around the sun.

______ is known to us all that the earth turns around the sun.

A. It B. Which

C. As D. That

前一句填C，as用以引導(dǎo)一個(gè)非限制性定語(yǔ)從句;后一句填A(yù)，it為形式主語(yǔ)，真正的主語(yǔ)是其后的that從句。

______ is difficult to persuade her to stay.

______ is difficult is to persuade her to stay.

A. It B. What

C. He D. That

前一句填A(yù)，it為形式主語(yǔ);后一句填B，what用以引導(dǎo)主語(yǔ)從句。注意后一句比前一句在不定式前多了個(gè)is。