比如说What he said.这里的What he said充当句子的主语，(is作谓语，correct作表语)，而这个名词性从句由what引导，所以是what引导的主语从句。
例句：
What she saw frightened her.
她看到的事情吓了她一跳。
What was once regarded as impossible has now become a reality.
曾经被认为不可能的事情，现在已经变成了事实。
What hurt our feelings most was the personal comment of the judge.
最伤我们感情的是法官的私下评论。
What is gone is gone.
过去的事就过去了。