it什么时候做形式主语 it做形式主语时后的动词用不定式还是ing

当不定式短语、动名词短语和从句作主语时，主语部分的长度总是超过谓语部分。在这种情况下，如果仍旧把主语放在谓语动词前，句子就会显得头重脚轻。为了调节句子的平衡，于是就把主语放在谓语部分的后面，原主语空缺的位置由形式主语it 充当“傀儡”，而真正的主语则在“后宫垂帘听政”。

形式主语用于三种情况。

　　1、如果表语是表示“好处”或“用处”的形容词或名词时，要用动名词短语作主语。常用的形容词有 good, helpful, helpless, useful, useless 等，名词有 no good, no use, no help 等。如：

• It is helpful having you in the room when major news is expected. 等待重大消息时最好有你们在场。

• It's nouse praying for the weather to change. 祈祷天气转变是徒劳的。

　　2. 如果表语是其他形容词或名词，则用不定式短语作主语。其结构是“It + 系动词be +表语 + (for sb.) to do sth”。如：

• It is important (for sb.) to do sth. （某人）做某事很重要。

• It is necessary for sb. to do sth. 某人）做某事必要的

• It was unfair to (for sb.) to do sth.（某人）做某事是不公平的。

• It was an effort (for sb.) to do sth.. （某人）做某事是件费劲的事。

• It mustbe a good job (for sb.) to do sth.. （某人）做某事是件好事。

　　除此之外，如果表示“付出时间、金钱或精力等做某事”，也要用不定式短语作主语，其结构是“It + take/cost + sb. + time/money/effort + to do sth”。如：

• It took/cost him two hours to do his homework. 做作业花了他两个小时。

• It took/cost me over $ 6 to buy the suit. 买那件衣服花掉我6英镑。

• It took him a hard work to pay off the debt. 还清这笔债务花费了他辛勤劳动。

　　【注：如果表语是 worth while，既可以用不定式也可以用动名词作主语。例如 It is worth while to discuss [discussing] the problem. 这个问题值得讨论一下。】

　　3、“It + 系动词be +表语 + for sb. to do sth”这种结构也可以改用主语从句，结构为“It + 系动词be +表语 + 主语从句”。如果强调“某人应该做某事”是说话人的个人看法，则从句需要用虚拟语气（should常常省略）。如：

• It is important that he (should) complete the task in time. 他及时完成该项任务是很重要的。

• It was necessary that they (should) come earlier. 他们很有必要提早一些来。

【it作形式主语常见句型】动词多用不定式

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 uncertainwhether 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 thatyou missed the exciting football match.
It remains aquestion 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 reportedthat 16 people were killed in the earthquake.
It is notdecided 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 thathe didn’t tell the truth.
It happened thatI 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 necessaryfor the young to master two foreign language.
It is unwise togive 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 kindof you to help me with the work.
=You are kind tohelp me.
It seemedselfish of him not to share his dictionary with others.
=He seemedselfish not to share his dictionary with others.

7. It + be +名词词组 + 动词不定式，如：
It is not a goodhabit 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 oftime talking to her any more.
It is no use arguingabout the matter with him.

9. It + take ( sb. )+ 时间（金钱）+ 动词不定式，如：
It took theworkers almost three years to finish building the dam.
It will take a wholeday to get to the top of the mountain on foot.