怎么用it做形式主语

当代英语中，通常用it 做形式主语，将不定式和动名词置于谓语后面，特别是主语较长时，或在一些习惯用法中。

1） 谓语是系表结构（be + 形容词 / 名词）时，常将不定式后置，构成句型 “it + be + 表语（形容词 / 名词）+ 不定式”。

例 1 It is my duty to care for that patient.

照料那位病人是我的职责。（做主语的不定式to care for that patient 后置，it 为形式主语）

例 2 It is not an easy thing to master a foreign language.

掌握一门外语不是件容易的事。

例 3 It isn’t right to speak ill of someone behind his back.

在别人背后说坏话是不对的。

2） 当谓语是 take，make，pay，cost ，require， feel，need 等动词时，或者是与情绪有关的动词 delight，amuse，excite，annoy，irritate 等时，习惯上常用 it 做形式主语，并将不定式后置。

例 1 It takes only ten minutes to get there.

到那儿只需十分钟。

例 2 It pays to be honest.

诚实是不会吃亏的。

例 3 It feels good to stay away from the crowded city.

能远离拥挤的城市，感觉真不错。

例 4 It annoyed her to see someone spit on the ground.

看见有人随地吐痰让她很生气。

3） 同不定式一样，动名词也可用于“it + be + 表语 + 动名词”句型中，二者的区别可参见10．2．2 中1）的论述。下面两组句子没有多大差异。

例 1 A） It is a waste of time to argue with him

B） It is a waste of time arguing with him.

和他争辩是浪费时间。

例 2 A） It is worthwhile to discuss this again.

B） It is worthwhile discussing this again.

这一点值得在讨论一下。

4） 但表语是（of）no use，not any use，not the slightest use， no good， not much good，fun等时，常用动名词（不用不定式）做主语并后置，用 it 做形式主语。

例如 It’s no use crying over spilt milk.

覆水难收。

5） 动名词还可在 there be 句型中做主语（不能用不定式），常见形式为 “there is no + 动名词”或 “there is no / any + 名词 + 动名词”。

例 1 There is no denying the fact.

事实无可否定。

例 2 There isn’t any use trying again.

再试也没用
参考:http://zhidao.baidu.com/question/9752246.html

It作形式主语常见句型
动词不定式，动名词或主语从句作主语时，通常把它们放在谓语动词前，而用it作形式主语，放到句首，常见句型有：
1. It + be + 形容词+主语从句，如：
It is uncertain whether he can come to Jenny’s birthday party or not.
2. It + be + 名词词组 + 主语从句，这类名词有：a fact, a pity, a shame, an honour，a question等，如：
It’s a pity that you missed the exciting football match.
It remains a question whether he will come or not.
3. It + be + 过去分词 + 主语从句，这类过去分词有：announced, believed, expected, hoped, decided, reported, said, shown等， 如：
It is reported that 16 people were killed in the earthquake.
It is not decided who will give the operation to the patient.
4. It + 不及物动词(seem, appear, happen等) + 主语从句, 这类不及物动词有：appear, happen, seem等， 如：
It seemed that he didn’t tell the truth.
It happened that I was out when he called.
5. It + be + 形容词 + for sb.）+ 动词不定式
这类句型常用形容词easy, difficult, hard, important, possible，wise等作表语，有时候为了强调不定式动作的执行者，常在不定式前加for sb，如：
It’s necessary for the young to master two foreign language.
It is unwise to give the children whatever they want.
这类形容词有：easy, difficult, hard, important, possible，wise
6. It + be + 形容词 + of sb. + 动词不定式，这类形容词常是表示心理品质,性格特征的形容词，如：kind, nice stupid, clever, foolish, polite, impolite, silly, selfish, considerate等。某人和这些形容词可以构成主系表结构。如：
It’s very kind of you to help me with the work.
=You are kind to help me.
It seemed selfish of him not to share his dictionary with others.
=He seemed selfish not to share his dictionary with others.
7. It + be +名词词组 + 动词不定式，如：
It is not a good habit to stay up too late.
8. It + be + 名词或形容词 + 动名词，这类名词和形容词常常是：good, no good, no use, a waste of, useless, senseless等，如：
It’s a waste of time talking to her any more.
It is no use arguing about the matter with him.
9. It + take ( sb. )+ 时间（金钱）+ 动词不定式，如：
It took the workers almost three years to finish building the dam.
It will take a whole day to get to the top of the mountain on foot.我的最准

It's ...not to do sth