主语不是一个词而是一个句子，它因为有这个主句了，所以主语这个句子叫从句。主语从句可以位于句首，也可以位于句子的尾部，而在句首一般用it做它的形式主语，引导主语从句的关连有that和whether

　　这两个都是不能省略的。

　　What she did is not yet know.

　　It is certain that we shell be late.

　　It is curt to him that he had forgot to buy the book.

　　How the prisoner is escape is a complete secret.

　　1. ______ at the meeting yesterday is of great importance to the development of our business.

　　A. That we discussed B. What we discussed

　　C. How did we discuss D. What did we discuss

　　答案 B

　　(be of importance= be important)

　　2. _______ you say now can't make up for what you've done.

　　A. No matter B. Whatever C. However D. That

　　make up for 为……补偿

　　答案 B

　　3. _______ was once regarded as impossible has now become a reality.

　　A. That B. Which C. Who D. What

　　答案 D

　　4. _______ you do， do it well.

　　A. What B. Which C. That D. Whatever

　　which不能引导主语从句。

　　whatever=no matter what

　　答案 D

　　5. __________ the prisoner escaped is a mystery.

　　A. What B. How C. That D. Which

　　引导主语从句的关联词是that，whether和疑问词。

　　答案 B