一、基本用法
當不定式、動名詞、從句等用作主語時，避免頭重腳輕，通常在句首使用形式主語it，而把真正的主語放在句子末尾：
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. 熬夜沒有好處。
It is quite clear that he has read the book. 很顯然，他讀過這本書。
It doesn't matter what he says. 他說什麼沒關係。
二、幾種特殊的形式主語
(1) 用作動詞look, seem, appear, happen, occur, follow等的主語(後接that從句或as if從句)：
It seems as if it is going to rain. 看來要下雨了。
It seems that he knows everything. 他好像什麼都知道。
It (so) happened that I had no money with me. 碰巧我沒帶錢。
From this witness it follows that he must be guilty. 根據該證人的證詞判斷，他一定有罪。
(2) 當系表結構後接有if 或when引出的狀語從句時，通常在系表結構前使用形式主語表示說話人對某情況的看法：
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. 如果出了什麼事就是一個悲劇。
(3) 用於其他固定句型，如if it were not for…(若不是因為)，it's time that…(該……了)等。
三、誤用形式主語的幾種情形
(1) 在該用it作形式主語的地方而誤用其他詞：
Is ______ necessary to tell his father everything?
A. it B. that
C. what D. he
此題的正確答案應是A(it用作形式主語)，而不能選D。
(2) 在不該用形式主語的地方而誤用形式主語：
______ must be something wrong with the machine.
A. There B. It
C. He D. That
此題應選A，為there be句型與情態動詞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用以引導一個非限制性定語從句;後一句填A，it為形式主語，真正的主語是其後的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，it為形式主語;後一句填B，what用以引導主語從句。注意後一句比前一句在不定式前多了個is。