形容词为necessary， important， impossible， strange， natural， essential时，在“It + be + 形容词或过去分词 + 主语从句”中，主语从句要虚拟。

It is necessary that the problem (should) be discussed at once.

立刻讨论这个问题很有必要。

It is strange that he (should) have so many friends. 太奇怪了，他有那么多的朋友。

6. 主语从句与强调句的区别

注意事项：

1)在主语从句中，把It was … that去掉之后，就已经完全不像一个句子了。

2)在强调句中，把It was … that去掉之后，还基本上像个句子。

3)主语从句有以下的引导词：what，who，whom，whose，which，whatever，whoever，whichever

4)强调句的引导词只有：that， who.

主语从句：

It was strange that he had made a mistake. 如果把，It was … that 去掉以后，剩下strange he had made a mistake.就不算是句子了。

It is immaterial where or when he goes. 他去何处或者何时去是无关紧要的。

It is said that…， 和It is reported that…这样的结构，后面接的是主语从句：

It is said that he‘s got married. 听说他已经结婚了。

强调句：

It was she that had been wrong. 错的是她。(强调主语)如果把，It was … that 去掉以后，剩下she had been wrong.还勉强算的上是一个句子。

It was this novel that they talked about last night. 他们昨晚谈论的是这部小说。(强调宾语)

It was in London that I first saw her. 我是在伦敦第一次见到她的。(强调地点状语)