二、主语从句与形式主语it

有时为了考虑句子平衡，通常在主语从句处使用形式主语it，而将真正的主语从句移至句末。这分三种情况：

(1) 对于以连词that引导的主语从句，通常用形式主语代主语从句：

It's a pity that he didn't come.
很遗憾他没来。

It is important that he should know about this. 他必须知道此事。

It's vital that we be present. 我们出席是至关重要的。

It was intended that you be the candidate. 大家的意图是让你当候选人。

It is important that this mission not fail. 这项使命不失败至关重要。

It is essential that a meeting be convened this week. 本周开一次会非常重要。

It is appropriate that this tax be abolished. 废除这个税是恰当的。

It's unfair that so many people should lose their jobs.
竟有这么多人失业这是不公平的。

It's amazing that she should have said nothing about it.她竟未谈及此事令人惊讶。

It's unthinkable that they should deny my request.
他们竟然拒绝我的请求，这是不可思议的。

That they should refuse to sign the petition required great courage.
他们拒绝在请愿书上签字这是需要很大勇气的。

(2) 对于以连接代词(副词)引导的主语从句，可以使用形式主语代主语从句，也可直接在句首使用主语从句：

Whether they would support us was a problem. 他们是否会支持我们还是一个问题。

It was a problem whether they would support us. 他们是否会支持我们还是一个问题。

It remains to be seen whether it will do us harm or good.
它对我们是利是害，还得看看再说。

(3) 对关系代词型what引导的主语从句，通常直接将主语从句放在句首。如：

What we need is money. 我们需要的是钱。

What I want to know is this. 我想知道的就是这一点。

有时也可将助用形式主语。如：

It is clear enough what he meant. 他是什么意思很清楚。

(4) 如果句子是疑问句，则必须用带形式主语it的结构：

Is it true that he is the girl's father? 他是那女孩的父亲，是真的吗？

How is it that you are late again? 你怎么又迟到了？

三、连词that的省略问题

引导主语从句的连词that 有时可省，有时不能省，其原则是：若that引导的主语从句直接位于句首，则that不能省略；若that引导的主语从句位于句末，而在句首使用了形式主语it，则that可以省略：

That you didn't go to the talk was a pity. 很遗憾你没去听报告。(that不可省)

It was a pity (that) you didn't go to the talk. 很遗憾你没去听报告。(that可省)