你好，这个句子是由主语从句转换来的
1）
在主语从句中,
为了防止句子头重脚轻，通常把形式主语it放在句首位置，真正主语搁置于句末
用it
作形式主语的结构
(1)
It
is
+名词+that从句
It
is
a
fact
that
…
事实是…
It
is
an
honor
that
…非常荣幸
It
is
common
knowledge
that
…是常识
(2)
it
is
+形容词+that从句
It
is
natural
that…
很自然…
It
is
strange
that…
奇怪的是…
(3)
it
+不及物动词+that从句
It
seems
that…
似乎…
It
happened
that…
碰巧…
(4)
it
is+过去分词+that从句
It
is
reported
that…
据报道…
It
has
been
proved
that…
已证实…
2）常规主语从句，即句子在复合句中充当一个主语
3）主语从句的时态：不受主句的时态影响和限制。
如：It
is
important
that
we
all
should
attend
the
meeting.
其实就是
that
we
all
should
attend
the
meeting
is
important
具体可以参考
http://baike.baidu.com/view/100914.htm希望帮到你