这是一个主语从句。而It's .....that是强调句
（1）主语从句。
It is important that students today master English.
It is reported that China will send up a series of spaceships in the 21st century.
It is a rule that everyone should obey the State’s laws.
It happened that they were out visiting their friends.
可以看出，此类主语从句的句子成分无论是主语、谓语、宾语，还是各种状语都是完整的，并且不能称到“It is…”结构中去。这就是判别主语从句的基本点。
（2）强调句。
先举一个重点成分句：昨天我在书店看见了他。I saw him at the bookstore yesterday.
现将此句变成分别突出主语、宾语、地点及时间状语的强调句：
A、It was I that/who saw him at the bookstore yesterday.
B、It was him that I saw at the bookstore yesterday.
C、It was at bookstore that I saw him yesterday.
D、It was yesterday that I saw him at the bookstore.
比较无重点叙述对象原句和由此转换来的强调句可以看出，强调句是把原句拆散，把要强调的部分如主语、宾语、状语移到“It is…”之后。如果把“It is…that”去掉，并把移走的部分放回到原位，句子还是完整的，这就是判别强调句的基本方法。再把主语从句和强调句加以对比；主语从句各成分不能移到“It is…”结构中，而强调句则可以把句子的主语、宾语、状语等成分移到“It is”结构后面，其余放在“that”后面。
如果“It is…”之后是地点状语、时间状语、介词短语或从句，“It is…that”必为强调句型。除上面所述的C、D例句外，再补充例证如下：
It was here/there that we took a picture of him last year.
It was last year he went abroad.
It was after I came here that I learned a lot.(我到此之后才学到很多东西。)
It was not until he came back that we had dinner.(直到他回来后我们才吃饭。)
另外，在强调句“It is…”中, it is后不会出现形容词、过去分词及谓语词形式，这些词用在带有主语从句的“It is…that-clause”结构中。
（3）“It is…that”结构中的that可能引出定语从句。
It is this dictionary that I am looking for everywhere.（这是我求之不得的词典啊。）
在这样的句型中，it不再是形式主语或无意义，而是说话人心目中的指代的“那个”。that是关系代词，介词for的后面乍看无宾语，实际是前移走了。又如：
It was the most fascinating place that we’ve visited.
