同意曾老师的看法，《张道真英语语法》在第513页也有类似说明：

绝大多数包含主语从句的句子都借助先行词 it 作为形式上的主语，而把主语从句放到句子后部去。这种先行主语后有四类常见的结构（以下为原文照录，但其中的下划线为本人所加）：

a. be+形容词+从句：

It’s strange that it hasn’t been noticed before. 很奇怪，以前没人注意到这一点。

It was plain to everyone that she was offended. 大家都能看出她是生气了。

How is it possible that a man can fall so low? 一个人怎么可能堕落到这个地步?

From the photographs it seems clear (that) she was no beauty. 从这些相片上看，显然她不是一个美人。

b. be+名词+从句：

It’s a shame (that) he didn’t come. 很遗憾他没来。

It was a fearful disappointment to us that you didn’t come to theparty. 你没来参加晚会使我们非常失望。

It’s a pity (that) he didn’t finish college. 真遗憾他大学没毕业。

It’s a wonder that he is still alive. 他还活着，真是奇迹。

c. 动词 (+宾语或状语) +从句：

It happened that the harvest was bad that year. 恰好那年收成不好。

It struck me that nobody was in favour of the change. 我突然感到没有人赞成这种改变。

It suddenly occurred to him that this was an act he could never do. 他突然想到这是他绝不能做的一件事。

It grieved her that George could not sleep there. 她很伤心，乔治不能在那里睡。

d. 动词的被动语态+从句：

It was agreed that the transaction should be kept secret. 双方同意这项交易应当保密。

It was felt that there had been some duplication of effort. 大家感到努力有些重复。

It is said that there has been an earthquake in Japan. 据说日本发生了一次地震。

It was reported that they had brought down two enemy planes. 据报导他们击落了两架敌机。

It happened that he was unfit for the office.
这个句子的 that 从句不是主语从句，而是表语从句。这里的 happened 可以用 seemed 替换。又如：
It happened that I was not in London at the time. 碰巧那时我不在伦敦。（《英语常见问题解答大词典》增订版）