Question

() is known to us all is that the 2008 Olympic Games will take place in Beijing.

A.it B.what C.as D.which为什么是选what,A C D为何不可，请详细，谢谢！！

Answer 1

1.() is known to us all是句子主语，称作主语从句。能引导主语从句，又在从句里做主语指事或物，用what。
2.(1)如果选A项it，则去掉第二个is才可以。这样变成了it作形式主语，that the 2008 Olympic Games will take place in Beijing是真正主语。It is known to us all that the 2008 Olympic Games will take place in Beijing.
(2)如果选C项as，则变成由as引导的定语从句，但要用逗号隔开.
即： As is known to us all , the 2008 Olympic Games will take place in Beijing.
(3)如果选D which，则也变成定语从句。which引导的定语从句不能位于句首

Answer 2

what is known to us all 这句话是句子的主语 () is known to us all is that the 2008 Olympic Games will take place in Beijing这整个句子是主语从句 前面是what引导的主语从句 此从句中又缺少主语引导词 故用what

Answer 3

因为这句话的陈述形式相当于：
that the 2008 Olympic Games will take place in Beijing is known to us all.
主语是： that the 2008 Olympic Games will take place in Beijing 可以用what指代这个主语。
所以别的选项不能起到这种指代的功能，不对。

追问

用it做形式主语呢？

追答

用it做形式主语的话，题干中的第二个is就是多余的了。
() is known to us all is that the 2008 Olympic Games will take place in Beijing.

Answer 4

What is known to us all 是名词性从句（主语从句）， 等于All that is known to us all , 等于The thing that is known to us all

Answer 5

因为后面是is that ....
用what is known to us all 做主语，表示我们都知道的是。。。