一、主语从句（subject clause）
在主句中用作主语的主谓结构称之为主语从句。
例句：
That he will come to the discussion is certain.
他来参加讨论是确定的。
That the moon moves round the earth is well known to all of us.
月球绕地球转动，这是我们大家都熟知的。
When the meeting is to be held
has not yet been decide.
会议什么时候召开还没有决定。
主语从句放在句首，句子常常显得比较笨重，因此通常可以把it放在句首，作形式主语，而将主语从句放在后面。例如上面的句子可以分别改写为：
It is certain that he will come to
the discussion.
It is well known to all of us that
the moon moves round the earth.
