2.形式主语it 有时为避免句子头重脚轻,常用it代替主语从句作形式主语放于句首,而把主语从句置于句末。主语从句后的谓语动词一般用单数形式。常用句型如下: It+be+名词/形容词/过去分词/不及物动词+that从句 It is a pity that you didn't attend the evening party. 你没参加晚会真是遗憾。 It is clear that light travels in straight lines. 很明显,光是沿直线传播的。