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.
這類從句一般相當於帶有定語從句所修飾的名詞詞組，即在結構上相當於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儘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.
上述各句中儘管都有一個以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。
（編輯：趙穎茹）