问一英文题目

这里有些重要的语言现象在里面：

1、当用it时，一般用主语从句，不能用逗号：

It is known to the world (that) Mark Twain is a great Americn writer.

2、当用as时，是前置的as引导的定语从句，中间要用逗号隔开：

As is known to the world, Mark Twain is a great Americn writer.

3、用as引导的定语从句还可以放在句末去，跟主句用逗号隔开：

Mark Twain is a great Americn writer，as is known to all.

4、像it is + pp. + that主语从句，用于这样的句子的过去分词还有hoped、believed、expected、thought、concerned等，但要注意一点，主语从句的内容要跟那个pp(过去分词)的意思保持连续性，不能跟它意思相违背，这时才能用as定语从句替换：

It is expected that he is admitted to Beijing University.
As is expected, he is admitted to Beijing University.
He is admitted to Beijing University, as is expected.

但下面的句子只能用it句式，不可用as句式，因为unexpected跟从句的内容相违背，出乎人们意料：
It is unexpected that he failed in the final exam, for you know he is good at his subjects.

(It )is known to the world (that)Mark Twain is a great Americn writer.
As is known to the world,(此处有逗号） Mark Twain is a grea