表语从句（predicative clause）

在主句中担当表语的主谓结构称之为表语从句。

One idea is that fish is the best brain food.有一种看法认为鱼是好的补脑食品。

The problem is where we can hold our meeting.问题是我们可以在哪儿举行会议。

My question is how information is stored in the long-term memory.我的问题是信息是怎样储存在长期记忆中的。

That is what he really wants.那就是他真想要的东西。

It seems that everything goes smoothly.似乎一切都进行得很顺利。

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。

例如：

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.

It has not yet been decide when the meeting is to be held.