It
is
+形容词+that
是一个句型，that
引导
主语从句
，it作
形式主语
指代后面的that从句。
that后面接
完整的句子
，that在从句中不做任何成分，没有实际意义。
比如It
is
amazing
that
he
came
to
ask
help
from
me.
他居然向我求助，真太令人吃惊了。
It
is
amazing
that
that
是主语从句
it是形式主语
例如：
It
is
amazing
that
he
has
passed
the
exam.主从复合句
It
is
amazing
是主句
it是形式主语
that
he
has
passed
the
exam
是主语从句
(真正的主语）
以上句子可以改写成以下句子：
That
he
has
passed
the
exam
is
amazing
.
That
he
has
passed
the
exam
主语从句作主语，
后面的谓语
is
amazing
是连系动词+表语
