第三類，用what、whatever、where、wherever、whoever、whichever等代詞引導的主語從句，例如：

What he said at the meeting encouraged everyone.

Whoever fails to see this will make a big blunder.

這類從句一般相當于帶有定語從句所修飾的名詞詞組，即在結構上相當于一個名詞加上一個定語從句，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.

切記，這類主語從句不可以使用先行it結構。

上述第二類主語從句與第三類主語從句盡管在形式上十分相似，但實際上是不同的。第二類結構中用連接代/副詞引導的從句系由特殊疑問句轉化而來，以whether/if引導的從句系由一般疑問句轉化而來，自然含有疑問的意味，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？

而第三類結構中的主語從句則沒有疑問的意味。

試比較下列各句：

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.

上述各句中盡管都有一個以what引導的主語從句，但其意義不盡相同；第①和③句中的主語從句系由“What caused the accident？”和“What does she look like？”轉化而來，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句中的主語從句含義分別為“The thing that caused the accident”和“The thing which she'd like”，所以不含疑問意味，因此不可以說：

It was a broken bottle what caused the accident.

It is a digital watch what she'd like.

但是可以說：

It was a broken bottle that caused the accident.

It is a digital watch that she'd like.

不過，這已不是主語從句，而是強調結構了。