1、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. 他説什麼沒關係。
2、幾種特殊的形式主語
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。
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説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。
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。
哇咔範
