（19）it的用法
it可用作人称代词、指示代词、先行词及引导词等。
1. 人称代词it，是第三人称单数中性，代表前文已提到过的一件事物。如：
1）That vase is valuable. It's more than 200 years old. 那个花瓶很珍贵，它有200多年的历史。
2）I love swimming. It keeps me fit. 我喜欢游泳，它能使我保持健康。
当说话者不清楚或无必要知道说话对象的性别时，也可用it来表示。 如：
3）It's a lovely baby. Is it a boy or a girl？ 宝宝真可爱，是男孩还是女孩？
it可用来指代团体。如：
4）The committee has met and it has rejected the proposal. 委员会已开过会，拒绝了这项建议。
it用以代替指示代词this， that.如：
5）—— What's this？ —— It's a pen. —这是什么？ —是一支钢笔。
6）—— Whose book is that？ —— It's Mike's. —那是谁的书？ —是迈克的。
2. 指示代词it，常用以指人。如：
7）Go and see who it is. 去看看是谁。
8）—— Who is making such a noise？ —是谁发出这样的吵闹声？
—— It must be the children. —一定是孩子们。
3. 虚义it无指代性，常用作没有具体意义的主语，出现于表示天气、气候、温度、时间、地点、距离等意义的句子中。如：
9）It is half past three now. 现在是三点半钟。
10）It is six miles to the nearest hospital from here. 这里离最近的医院也有六英里。
11）It was very cold；it snowed and grew dark. 天气很冷；天下着雪，渐渐地变黑了。
it也常用来表示一般的笼统的情况。如：
12）It's awful—I've got so much work I don't know where to start. 糟透了——我有这么多工作要做我不知从何开始。
13）How is it going with you？ 你近况如何？
14）Take it easy. 不要紧张。
it也常用于下列结构：
15）It looks as if the college is very small. 看起来这个学院很小。
16）It seems as though our plan will be perfect. 似乎我们的计划很完善。
17）It's my turn. 该轮到我了。
it也常用于某些习惯用语中作宾语，各该习惯用语有具体意义，但it并无具体意义。如：
cab it 乘车 catch it 受责，受罚 come it 尽自己分内 come it strong 做得过分 walk it 步行
make it 办成 take it out of somebody 拿某人出气
4. 先行词it.
it充当形式主语或形式宾语，本身无意义，只起一种先行引导的作用，先行词不重读。后面的真正主语或真正宾语通常是不定式结构、-ing分词结构或名词性从句。
（1）用作形式主语
18）It is difficult to translate this article. 翻译这篇文章很难。
19）It is getting harder every day for a lazy man to get a living. 懒汉谋生是日益困难了。
20）What time would it be most convenient for me to call again？什么时候我再给你打电话最合适？
21）It is no use going there so early. 这么早去那里没有用。
22）It has been a great honour your coming to visit me. 你的来访是我很大的荣幸。
23）It is strange that he did not come at all. 真奇怪，他竟还没来。
24）It doesn't matter what you do.什么都没关系。
在上述诸例中，后置的真正主语可以取代先行it的位置，而出现于句首。
To translate this article is difficult.
Going there so early is no use.
That he did not come at all is strange.
上述结构远不如用先行it结构普通。此外，也并非所有先行it结构都可作上述转换。如：
25）It seems that John is not coming after all. 似乎约翰终竟不来。
26）It happened that John was the only witness. 碰巧约翰是唯一的证人。
上述两例只能转换为：
John doesn't seem to be coming after all.
John happened to be the only witness.
（2）用作形式宾语
27）I found it difficult to explain to him what happened. 我觉得向他解释清发生了什么事很困难。
28）He thought it no use going over the subject again. 他认为再讨论这个问题没有用了。
29）They kept it quiet that he was dead. 他们对他的死保密。
30）We must make it clear to the public that something should be done to stop pollution.
我们必须使公众明白应该采取措施制止污染。
5. 分裂句引导词it.
分裂句是以it为引导词的强调句型。它的结构形式是：It+be的一定形式+中心成分+that-/who-从句。如：
31）It was Jane that/who called this morning. 是珍妮今天上午打来电话。
32）It was a parcel that she brought him. 她带给他的是一个包裹。
分裂句是由普通陈述句转换而来的。通过这种结构可以强调除谓语动词以外的大多数句子成分。如：
33）John gave Mary a handbag at Christmas. 约翰在圣诞节给了玛丽一个提包。
根据上下文和语义意图，说话人可以通过分裂句分别强调主语、间接宾语、直接宾语、状语，使之成为信息中心。
34）It was John that/who gave Mary a handbag at Christmas. 是约翰在圣诞节给了玛丽一个提包。
35）It was Mary that John gave a handbag （to） at Christmas. 约翰在圣诞节是给玛丽提包的。
36）It was a handbag that John gave Mary at Christmas. 约翰在圣诞节给玛丽的是一个提包。
37）It was at Christmas that John gave Mary a handbag. 是在圣诞节约翰给了玛丽一个提包。
分裂句的时态一般应一致，即主句与从句的时态应皆用现在时，或皆用过去时，或皆用将来时。如：
38）It is not I who am angry. 发怒的不是我。
39）It was my two sisters who knew her best. 是我的两个姐妹最了解她。
40）It will not be you who will have to take the blame for this. 对此须受责难的将不是你。
有时分裂句的谓语动词还可以采取复杂形式。如：
41）It may have been at Christmas that John gave Mary a handbag. 可能是在圣诞节约翰给玛丽一个提包。
42）It might have been John who gave Mary a handbag. 很可能是约翰给了玛丽一个提包。
分裂句可以强调多种多样的状语成分。如：
43）It was when she was about to go to bed that the telephone rang. 是在她即将上床睡觉时电话铃响了。
44）It was because I wanted to buy a dictionary that I went to town yesterday. 我昨天是由于想买一本词典而进城的。
45）It was not until his father came back that Tom went to bed yesterday. 昨天汤姆是直到他父亲回来才上床睡觉的。（注意句中not的位置）
主语补语通常不可以用作分裂句的中心成分，如在规范英语中通常不说：
It is beautiful that she is.
It is chairman of the Committee that he is.
但是宾语补语是可以这样用的。如：