主语从句可以按其引导词的不同分为三类：
第一类,用从属连词
that
引导的主语从句
,例如：That we shall be late is certain.
That the driver could not control his car was obvious.
这种结构主要是对
that
从句的内容进行强调
,属正式文体
,连词that
不可以省略
；但是在一般情况下
,往往使用先行
it结构,即用it作形式主语
,而把that
从句放到后面
,这时,在口语中
,连词that
有时则可以省略
.所以上述两句可以改为：
It is certain that we shall be late.
It was obvious that the driver could not control his car.
如果整个句子是疑问形式
,就只能用先行
it结构,例如：Is it true that he would take the risk?
Is it possible that they will come tomorrow?
常使用先行
it结构,用that
从句作主语的句子 ...