it用法比较复杂，但可以总结如下：

一. It引起的几个易混淆的时间句型

1）It + be + 时间 + since-clause:

这个句型表示从since 从句谓语动作发生以后到现在或过去所经过的一段时间，意为“自从......以来已多久了”，主句多用一般现在时，从句用一般过去时，如果表示过去的情况，主句用一般过去时，从句用过去完成时，或主句用过去完成时，从句用一般过去时。例如：

It is three years since his father passed away . (从现在算起)

It was 10 years since they married . (从过去算起)

It had been fifteen years since she left Shanghai . (从过去算起)

2）It + be + 时间 + before-clause:

这个句型中的时间一般为表示一段时间的词语(如：long years , months , weeks , days ,hours , minutes ) ，主句中的谓语动词用肯定式，意为“过多长时间才......”。主句的谓语动词是否定式时，意为“没过多长时间就......”。主句的时态可用过去时was或将来时will be；用was 时，before从句的动词用一般过去时；用will be 时，before从句常用一般现在时。 例如：

It was long before the police arrived .

It will be hours before he makes a decision .

It will not be hours before meet again .

3）It + be + 时间 + when-clause:

这个句型中, it 指时间，而且表示时间的词语前没有介词(时间一般为具体时间)。主句中的谓语动词和从句中的谓语动词在时态上一般是一致的，主句是will be，when从句用一般现在时代替将来时。

It was already 8 o'clock when we got home .

It was the next morning when we finished our work .

It will be midnight when they get there .

4） It + be + 时间 （地点等被强调的部分）+ that-clause

这个句型是个强调句型。

例如： It was two years ago that he made an important invention .

(原句是：He made an important invention two years ago . )

It was at 5 o'clock that he practised playing the violin in the morning .

(原句是：He practised playing the violin at 5 o'clock in the morning . )

It was not until the next morning that he appeared in his office again.

(原句是：He didn’t appear in his office again until the next morning.)

5）It + be + time + that-clause

这个句型属虚拟语气结构，不管主句中用的是 is 或 was ，that从句都须用动词的过去式或should + 动词原型(但不及用过去式普通)，在time之前有时可加上high或about 以加强语气。例如：

It is time(that)we handed in our exercises .(= It is time that we should hand in our exercises .)

It is high time (that ) she wrote a letter to her boyfriend .

6）It / This / That + the first(second , third …) time + that-clause:

这个句型表示截止到说话时为止的某人的一种经历，关键是注意time前有序数词，主句是 is 时 ，从句要用现在完成时；如果主句用一般过去时was ，则从句须相应地用过去完成时。例如：

This is the first time (that) these Europeans have visited the Great Wall .

It was the fifth time (that) I had paid a friendly visit to Africa .

二、It代替从句作形式主语的常见句型

1. it 代替连词 that 引导的从句作形式主语。

1) it + be + 过去分词 + that 从句:

It's said that Tom has come back from abroad .

It was reported that dozens of children died in the accident .

可用于该句型的过去分词还有：known , thought , told , believed , hoped , announced ,expected , decided , suggested等，该句型常可以转换成含有宾语从句的复合句或者含有不定式作主补的简单句。如：

It's said that Tom has come back from abroad .

→People say that Tom has come back from abroad . →He is said to have come back from abroad .

2) It + be + 名词词组 + that 从句:

It's a pity that he can't swim for his age .

It's a wonder that you could finish such a hard task in such a short time .

可用于该句型的名词词组还有：an honour , a good thing , a fact , a surprise , good news等。

3) It + be /seem + 形容词 + that 从句:

It is likely that they will beat us tonight .今晚很可能他们会赢我们。

It doesn't seem that she will give us a hand .她帮我们的可能性似乎不太大。

可用于此句型的开容词还有；wonderful , true , good , right , wrong , (un)important ,useless , surprising , clear , possible , unusual , lucky , certain 等。

4) It + seem / appear / happen + that 从句:

It seems that he enjoys pop songs very much .看来他非常喜欢流行歌曲。

It appears that Tom might change his mind .看来汤姆会改变主意。

It happened that I went out that day .碰巧我那天外出了。

2. it 代替连接代词或连接副词引导的从句作形式主语。

1) It is known (not decided , not made clear , found out , discussed , being discussed , being considered , a question , uncertain等) + 连接代词或连接副词引导的从句。

It is still a question when we shall have our sports meet . 我们将什么时候举行运动会还是一个问题。

It is uncertain whether he can attend this conference or not . 他能否来开会还不能确定。



2) It doesn't matter(doesn't matter too much , doesn't make too much difference等) + 连接代词或连接副词引导的从句。如：

It doesn't matter whether we go together or separately . 我们一起去还是分开去都可以。

It doesn't matter too much to me what you do or where you go . 你做什么或去什么地方，对我都无关紧要