哪些从句作主语时可用it作形式主语
原则上说，it用作形式主语可以代替下列从句：
1. that从句
It is quite clear that he has read the book. 很显然，他读过这本书。(HM)
2. what从句
It doesn’t matter what he says. 他说什么没关系。(HM)
It was clear enough what he meant. 他的意思是很明显的。
3. who从句
It hasn’t been decided who will be sent to work there. 还没决定将派谁到那里工作。
4. how从句
It struck her how gentle he was being. 她深深感到他多么温存。
5. when从句
It hasn’t been made clear when the conference is to take place. 会议什么时候召开还没有宣布。
6. where从句
It’s not known where she went. 她到哪里去了没人知道。
It did not matter much where he lived. 他在哪里住都没有关系。
7. why从句
It was clear why he had asked for a conference. 他为什么要求召开会议原因很清楚。
8. whether从句
It makes little difference whether we go or stay. 我们去还是留没有多大差别。
It’s not yet settled whether I am going to America. 我去不去美国还没定。