1.用it作形式主语的that-从句有以下四种不同的搭配关系：
a. It + be +形容词+ that-从句
It is necessary that… 有必要……
It is important that… 重要的是……
It is obvious that… 很明显……
b. It + be + -ed 分词+ that-从句
It is believed that… 人们相信……
It is known to all that… 从所周知……
It has been decided that… 已决定……
c. It + be +名词+ that-从句
It is common knowledge that… ……是常识
It is a surprise that… 令人惊奇的是……
It is a fact that… 事实是……
d. It +不及物动词+ that-分句
It appears that… 似乎……
It happens that… 碰巧……
It occurred to me that… 我突然想起……
对应例句研习：
It is necessary that we go to help her.
我们去帮助她是必要的。
It is important that he should attend every day.
他每天都要出席，这很重要。
It was obvious that money flowed freely in their family.
显然他们家族有的是钱。
It was patently obvious that she was lying.
她显然是在撒谎。
It's obvious that she adores him.
她显然深深地爱着他。
It is believed that the couple have left the country.
据信那对夫妇已经离开了这个国家。
It was believed that people could be possessed by evil spirits.
以前，人们相信人有可能被恶魔缠身。
It was widely believed that Powell had stampeded the Government into taking action.
大家普遍认为鲍威尔已经逼迫政府采取行动。
It is known to all that man can't live without air or water.
众所周知，人离开空气或水就不能生存。
It is known to all that Shanghai has a modern and fashionable atmosphere.
众所周知，上海拥有时尚和现代化的氛围。
It is known to all that these islands have always been under Chinese jurisdiction.
众所周知，这些岛屿向来归中国管辖。
It has been decided that the meeting ( should) be put off.
已经决定推迟这个会议。
It has been decided that the meeting ( should) be held tomorrow.
已经决定会议明天举行。
It has been decided that we go by train.
我们坐火车去的已决被定下来了。
It is common knowledge that swimming is one of the best forms of exercise.
大家都知道游泳是最好的锻炼方式之一。
It is common knowledge that Zhang Zoulin was murdered by the Japanese.
尽人皆知，张作霖是被日本人谋杀的。
It is a surprise that you gave such an answer to the question.
令人惊奇的是你对这个问题作了这样的回答。
It is a surprise that the house was built on the top of the mountain.
那座房子建在山顶上，真令人惊奇。
Isn't it a fact that the firm is losing money?
公司正在亏本，这难道不是事实吗？
It is a fact that Chinese is being accepted as an international language.
汉语正在被人们接受为一种国际语言，这是一个事实。
2.Wh-从句作主语也常用先行词it做形式主语，而将wh-从句置于句末，例如：
It is not yet decided who will edit it.
由谁校订未决。
It is not yet decided who will be sent to work there.
还没决定将派谁到那里工作。
It is not yet decided when and where the test will be given.
何时何地举行这次考试，还没有定下来。
It is not yet decided who will do that job.
还没决定谁做这项工作。
It's unknown when the new law will take effect.
这项新规定何时生效还不知道。
It was unknown when or how the fire started.
何时或如何开始防火依然还是未知数。
It remains unknown when they are going to get married. 他们何时结婚依然不明。
It is still unknown when the tower was built.
那座塔是什么时候建的无人知晓。