第三类，用what、whatever、where、wherever、whoever、whichever等代词引导的主语从句，例如：

What he said at the meeting encouraged everyone.

Whoever fails to see this will make a big blunder.

这类从句一般相当于带有定语从句所修饰的名词词组，即在结构上相当于一个名词加上一个定语从句，例如：

What he wants to buy is a ten-speed bicycle.

→ The thing that he wants to buy is a ten-speed bicycle.

Whoever breaks the law is sure to be punished.

→ Anyone who breaks the law is sure to be punished.

切记，这类主语从句不可以使用先行it结构。

上述第二类主语从句与第三类主语从句尽管在形式上十分相似，但实际上是不同的。第二类结构中用连接代/副词引导的从句系由特殊疑问句转化而来，以whether/if引导的从句系由一般疑问句转化而来，自然含有疑问的意味，例如：

When they will have the sports meet is still a question.

→ When will they have the sports meet？

Who he is doesn't concern me.

→ Who is he？

Whether he will join us won't make too much difference.

→ Will he join us？

而第三类结构中的主语从句则没有疑问的意味。

试比较下列各句：

① What caused the accident is a complete mystery.

② What caused the accident was a broken bottle.

③ What she looks like doesn't matter.

④ What she'd like is a digital watch.

上述各句中尽管都有一个以what引导的主语从句，但其意义不尽相同；第①和③句中的主语从句系由“What caused the accident？”和“What does she look like？”转化而来，所以可以改成：

It is a complete mystery what caused the accident.

It doesn't matter what she looks like.

第②④句中的主语从句含义分别为“The thing that caused the accident”和“The thing which she'd like”，所以不含疑问意味，因此不可以说：

It was a broken bottle what caused the accident.

It is a digital watch what she'd like.

但是可以说：

It was a broken bottle that caused the accident.

It is a digital watch that she'd like.

不过，这已不是主语从句，而是强调结构了。