it引导的表语从句的几种句型

it不能引导表语从句。
用作表语的从句叫做表语从句(predicative clause)。引导表语从句的关联词有疑问代词、疑问副词、缩合连接代词、从属连词等。如：
The problem is who we can get to replace her. 问题是我们能找到谁去替换她呢。(关联词是疑问代词who)
The question is how he did it. 问题是他是如何做此事的。(关联词是疑问副词how)
That was what she did this morning on reaching the attic. 那就是她今晨上了阁楼干的。(关联词是缩合连接代词what)
He looked just as he had looked ten years before. 他看起来还与十年前一样。(关联词是从属连词as)
从属连词that有时亦可引导表语从句，如：
The trouble is that I have lost his address. 麻烦是我把他的地址丢了。
从属连词whether有时亦可引导表语从句，如：
The question is whether they will be able to help us. 问题是他们是否能帮我们。
[注] 从属连词if一般不用来引导表语从句，但as if却可引导表语从句，如All this was over twenty years ago, but it’s as if it was only yesterday.(这都是20多年前的事了，但宛如昨天一样。)