一、考查 it 用作形式主语的用法

当不定式、动名词、从句等用作主语时，为避免头重脚轻，通常在句首使用形式主语it，而把真正的主语放在句子末尾。如：

It’s no good sitting up too late. 熬夜没有好处。

It is quite clear that he has read the book. 很显然，他读过这本书。

It doesn’t matter what he says. 他说什么没关系。

【特别提醒】注意it用作形式主语的三类特殊句式：

1.用作动词look, seem, appear, happen, occur, follow等的主语(后接that从句或as if从句)。如：

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

It (so) happened that I had no money with me. 碰巧我没带钱。

2.当系表结构后接有if 或when引出的状语从句时，通常在系表结构前使用形式主语表示说话人对某情况的看法。如：

It was a great surprise to me when he did a thing like that. 他这样做使我很吃惊。

It would be a tragedy if anything went wrong. 如果出了什么事就是一个悲剧。

3.用于if it were not for… / if it hadn’t been for…(若不是因为)。如：

If it hadn’t been for the freezing wind they could have reached the injured man in time. 要不是有刺骨的寒风，他们就及时赶到伤员身边了。