it做形式主语,哪些形式可做真实主语

所谓的头重脚轻就是主语很长而后面谓语宾语补语等等都很短,为了避免这样,要把主语变成形式主语,一般是it,真正的主语在后面

同学,很高兴回答你的问题才看到你发来的求助题,所以回答晚了些正确答案：只看这一句话,并不能说it 是形式主语,如Here is a present.It is only for you.这有一个礼物,它只是为你买的.这句话中的it 指代present,就是...

形式主语是it,真正主语是to travel everywhere by plane
常用的句型是it is +adj.+to do sth

主语从句(Subject Clause)
定义:在复合句中充当主语的句子叫做主语从句.
第一部分：常规主语从句,即句子在复合句中充当一个主语
主语从句的时态：不受主句的时态影响和限制.
（1）That he finished writing the composition in such a short time surprised us all.
（2）Whether we will go for an outing tomorrow remains unknown.
（3）Who will be our monitor hasn't been decided yet.
（4） Whom we must study for is a question of great importance.
（5）What caused the accident remains unknown.
（6）Whatever you did is right.
（7）Whose watch was lost is unknown.
（8）What we need is time.
（9）What we need are good doctors.
小结:
（1)引导主语从句连词有that,whether,who,what,whatever等
（2)连词位于句首不能省略
（3)主语从句大多数情况下视为三单,但也有例外,如例9
第二部分：为了防止句子头重脚轻,通常把形式主语it放在主语位置,真正主语搁置于句末
（1）It is certain that he will win the match.
（2）It is true that he has made a very important discovery in chemistry.
（3）It is very likely that they will hold a meeting.
（4）It is strange that he should do that.
（5）It is important that we all should attend the meeting.
（6）It is strange that the man should have stuck to his silly ideas.
（7）It is a pity that we won't be able to go to the south to spend our summer vacation.
（8）It is still a mystery what caused the accident.
（9）It is said that he has gone to shanghai.(=He is said to have gone to shanghai)
（10）It is known to all that the gun powder was first invented by the Chinese.
（11）It is suggested that the work should be done with great care.
（12）It seems that he has seen the film.(=He seems to have seen the film)
（13）It happened that the two cheats were there. (=The two cheats happened to be there)
小结:
（1)以that 引出的主语从句,常以形式主语it引导. It is +形容词/名词/某些动词ed + that 从句.
（2) 在有些that从句中要用虚拟语气 (should+do/should+have done)例句4,5,6,11.
主语从句与宾语从句两者的用法和在句中的作用是不同的,下面从它们的各自用法,介绍：）~
一．主语从句
主语从句 定义:在句子中担当主语的是一个从句,这个从句就叫做主语从句.
主语从句是在复合句中充当主语的从句,通常放在主句谓语动词之前或由形式主语it代替,而本身放在句子末尾.
1. It 作形式主语和it引导强调句的比较
It 作形式主语代替主语从句,主要是为了平衡句子结构,主语从句的连接词没有变化.而it引导的强调句则是对句子某一部分进行强调,无论强调的是什么成分,都可用连词that.被强调部分指人是也可用who/whom.例如：
It is a pity that you didn’t go to see the film.
2. 用it 作形式主语的结构
(1) It is ＋名词＋从句
It is a fact that … 事实是…
It is an honor that …非常荣幸
It is common knowledge that …是常识
(2) it is ＋形容词＋从句
It is natural that… 很自然…
It is strange that… 奇怪的是…
(3) it ＋不及物动词＋从句
It seems that… 似乎…
It happened that… 碰巧…
(4) it is＋过去分词＋从句
It is reported that… 据报道…
It has been proved that… 已证实…
3. 主语从句不可位于句首的五种情况
(1) if 引导的主语从句不可居于复合句句首.
(2) It is said , (reported) …结构中的主语从句不可提前.例如：
It is said that President Jingo will visit our school next week.
(3) It happens…, It occurs… 结构中的主语从句不可提前.例如：
It occurred to him that he failed in the examination.
(4) It doesn’t matter how/whether …结构中的主语从句不可提前.例如：
It doesn’t matter whether he is wrong or not.
(5) 含主语从句的复合句是疑问句时,主语从句不可提前.例如：
Is it likely that it will rain in the evening?
4. What 与that 在引导主语从句时的区别
What 引导主语从句时在从句中充当句子成分,如主语．宾语．表语,而that 则不然.例如：
What you said yesterday is right.
二：宾语从句的几个特征：1、引导词：whatwhichwhosewhenwhetherifwhere
2语序：宾语从句必须是用陈述语句.
如：I think that you must work harder.
宾语从句的引导词、连接词的区别、否定转移等现象.
(1)表达时间的几个句型：一般用when或者是what time：
What time will the train leave?
由于时刻都是固定的,也可以用一般现在时代替一般将来时：
What time does the train leave?
(2)时间的表达方式：8：19(nineteen past eight),7:57(three to eight),在时间的中间也可以加上分的单词(minutes)
(3)had better +动词原型.意思是提要求,建议.但是上下级不能用这种方式,因为语气太重：
You had better give up smoking .
(4)sb leave sth +地点
I left my book in my classroom yesterday.
(5)may I have your name(addressage)?一般询问对方的年龄、名字等,不要习惯以前的问法,这样很不礼貌,而是要采访上面的问法.以前的习惯是：how old are you? what's your name?
(6)修饰名词的代词次序：限-描-大-形---年---颜----籍----物--------类+名词：
this is a bridge.
This is a beautiful bridge
补充：从句的语序永远是陈述句.

英语中,形式主语和逻辑主语是完全不一样的.形式主语指的是用it代替作主语的动词不定式、动名词、主语从句.主要是为了保持句子的平衡.,逻辑主语是非谓语动词的动作的执行者或承受者；它们在逻辑上存在着主谓关系或动宾关系,但不能直接作非谓语动词的主语,因此叫逻辑主语.

1. To refuse him is not easy this time.
2. It is not easy to refuse him this time.
这两句话的表达都是正确的,并且主语都是动词不定式to refuse him.
而第二句话才是您所说的情况.
呵呵

和it一样
that is adj for sb to do sth
个人感觉that更多用于口语

这不是形式主语的句子
这个是表语从句
用that 引导的表语从句,that 在从句里不做成分

希望我的回答对您有帮助!

It is useless for her crying over my death.
我死了她的哭泣毫无意义.

形式主语是动词不定式的一种,它也是逻辑主语,可以把它归类到逻辑主语的第一种(不定式的逻辑主语) 用作宾语的不定式,用作目的状语,用作结果状语的不定式都是属于不定式的大概念里面的.
形式主语it
It+ is/was+形容词/名词+不定式
It+ is/was+形容词/名词+ 动名词
It+ is/ was+形容词/名词+ that从句
例句：
在公共场所大声讲话是不礼貌的.
It is impolite to talk loudly in public places.
吃太多朱古力没什么好处.
It is no good eating too much chocolate.
"it"做形式主语的用法,详细的可以看这里：

B 理由：It takes sb sometime to do sth
D 理由：it is +形容词+for sb to do
C 理由：sb find it +形容词+to do
C 理由：think it+形容词+to do
C 理由：it is +形容词+ to do

在英语里it等可以作为形式主语,由that引导的从句是实际主语.一般用形式主语是为了避免头重脚轻.（有点简陋,希望能满意）

it作形式主语,形式宾语的用法
☺ it 作形式主语
1.他犯了那样一个错误真是遗憾.
That he made such a mistake is a pity .
It is a pity that he made such a mistake.
It is well known/reported/ thought/said that…
It is clear/necessary/certain/true that…
It is a pity/a shame/an honor that…
It doesn’t matter whether…
It seems that…
It happens that…
1.____is known to all,Taiwan is only part of China.
A.As B.That C.Which D.What
2.It is known to all____ Taiwan is only part of China.
　　A.as B.that C.if D.for
答案：AB
☺.it 作形式宾语
(1) make /find/ feel / consider / think it + adj / n + that… / to do…
(2) it 指代后面从句所叙述的内容,常用于这些动词后,like/dislike/love/hate /appreciate/make+it 后常跟if 或when从句
I hate it when people laugh at the old
(3) depend on it that….
1.I like ___ in the autumn when the weather is clear and bright.
A.this B.that C.it D.one
2.You may depend on ____ that you parents will help you whenever you need it.
A.them B.yourself C.it D.me
答案：CC

当it 做形式主语时 通常是表述一个客观事实,一般用现在时 不能用过去时

Ⅰ.动名词和不定式做主语的相同点
1.不定式短语和动名词做主语时,常常为了保持句子的平衡,用it来做形式主语,如：
It is not easy to learn English well.
It is no use asking him for help.
2.如果主语用了不定式,表语和宾语也要用不定式;主语用了动名词,表语和宾语也要用动名词,这在英语中称为对称性,如:
To see is to believe.= Seeing is believing.(眼见为实)
It is better to stay home than _A__ out.
A.to go B.going C.go D.gone
Ⅱ.动名词和不定式做主语的不同点
1.不定式做主语常表示具体的某一动作;表示泛指或一般的抽象的概念时,多用动名词,如:
_B_ is a good form of exercise for both young and old.
A.The walk B.Walking C.To walk D.Walk
2.在”There/It +be+ no use(good/help/need)”之后常用动名词做主语,而少用不定式,如:
It /There is no use crying over the split milk.
It /There is no help taking this medicine.
★但是如果该句型中用了for引出不定式的逻辑主语,就只能用不定式,如:
There is no good for us to do that exercise.
3.不定式可以和when,where,how,what,whether等连用,在句子中做主语,宾语和表语,而动名词则不能,如:
How to solve the problem is a hot potato.
Where to put the box seems hard to decide.
Whether to do it hasn’t been decided.
What to do next is up to you.

形式主语的话 你换过来试试 变成 SLEEP IS TIME. 是不对的 而且 一般形式主语的句式也应该是IT IS + ADJ 形容词 TO DO. time不是形容词 这里的IT 你可以理解为 现在这个时间点
本人不是语法高手 解释的不一定特别清楚 希望对你有帮助
也欢迎语法高手来帮忙

“It” 用法及其句型和固定搭配,是英语语法的重点、难点,又是近几年考试的热点,因此在复习中应给予足够的重视.现将it用法归纳如下：
一、It用作实词
表达以下概念:指代前文提到的事物,前文中的this, that；替代前文中的内容；指代一位性别不明的小孩或未知的人；指代未指明但谈话双方都知道的那件事；指代时间、天气、气候、距离等自然现象.
二、It用作形式主语
替代作主语的从句、动词不定式、动名词,而把真正作主语的从句、动词不定式、动名词置于句尾.
It 作形式主语的常见句型：1. 代作主语的动词不定式,其句型为（1）It be adj. (for sb.) to do sth.
（2）It be adj. of sb. to do sth.
（3）It替代作主语的动名词的常见句型：
It’s no good/use doing…
It’s (well)worth doing…
It’s (well)worth one’s while doing/to do It’s (well)worth while doing/ to do
2. It替代作主语的从句常见句型
（1）It is + noun +从句
（2）.It is adj. +clause
It’s surprising that… (should)………竟然……
It’s a pity/shame that…(should)………竟然…
（3）It verb sb. + clause= It is v-ing + clause

It+surprise/delight/interest/disappoint/worry/disturb/annoy/amaze/bother/concern/frighten/please/anger sb. that…
（4）It verb (to sb.) that…= sb/sth verb to do
(verb = appear, seem, come about, emerge, follow, chance, happen, occur, transpire, turn out , work out)
（5）It is v-ed that…=sb/sth is to do
(verb=say, report, think, believe, hope, expect, agree, accept, decide, determine, intend, plan, understand, know）
四、It 作形式宾语
用来替代作宾语的从句、动词不定式、动名词,而把真正作宾语的从句、动词不定式、动名词置于句尾.
It 作形式宾语的常见句型：
1. verb+ it+ adj./noun (for/of) to do/clause (verb=think, believe, suppose, consider, feel, make, keep…)
五、强调句型
It is/was+被强调部分+ that(who)… 强调句型用来强调谓语动词以外的任何句子成分.当被强调部分是人时也可以用who.
在使用强调句型时需注意以下几点：
1. 请注意强调句型的特殊疑问句
2. 在强调原因状语从句时,只能强调由because所引导的从句.
3. 在强调not … until结构时必须把not与until一起放到被强调的位置上.
4. 注意强调句型与定语从句的区别.
六、It 常用的固定搭配

　　形式主语是动词不定式的一种,它也是逻辑主语, 可以把它归类到逻辑主语的第一种（不定式的逻辑主语） 用作宾语的不定式,用作目的状语,用作结果状语的不定式都是属于不定式的大概念里面的.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”.

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.
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 　　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.
　　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”.
　　 take 的用法　　
还不错.

楼上的答非所问啊呵呵
楼主问的是除了that可以引导从句外还有什么词可以 你回答的是 形式主语的定义 （并用不定式和从句举例）
It is my pleasure to answer your question.
这是不定式做逻辑主语 it作形式主语 逻辑主语可以是不定式 动名词以及从句
下面是你要的答案
It is obvious that I am glad to answer your question .
这是that引导从句
It remains unknown when I will answer your quertion.
when 引导从句
It depends on the weather whether we will go.
whether引导主语从句
所以得出结论 不止that可以引导从句

是的.
It是形式主语,真正的主语在后面,是动词不定式to help me.
kind of you里面kind 和you是主谓关系.如果是非主谓关系就只能用for.
EG:It's very important for us to learn English well.
这句话中to learn English well就是主语.而important和us就不是主谓关系.

可以的
it用作形式主语时,可以代替由
连词：that,whether；
连接代词：who,whom,whose,what,which；
连接副词：when,where,why,how；
等构成的主语从句而放在句首,这时候的主语从句放在句尾.
如：Whether Tom will come is uncertain.= It is uncertain whether Tom will come .
翻译为：不太确定Tom是否回来.
When Jack will come is not known.= It is not known when Jack will come.
翻译为：不太清楚Jack什么时候过来.
对你有所帮助的话,

1.本句是强调句式.it is...that...
2.主语是the demand 和the desire . 谓语是 determine

当主语、宾语是很长的从句、不定式、动名词时常用“形式主语”和“形式宾语”来替代,而真正的主语、宾语置于句末.
1.用“形式主语”的情况下,说明真正的主语（从句、不定式、动名词）较长, 用“形式主语”是比较符合英语习惯的.不用也可,但句子显得头重脚轻.如：
That he won the game is true.
不如It is true that he won the game .
2.在要用“形式宾语”的情况下,“形式宾语”是必须要用的,如：
I find it hard to get along with her.
不可把to get along with her放在find 和 hard 之间.

1.做主语
不定式与动名词作主语一般来说区别不大,有时两者可以互换.
一般来说,动名词表示抽象动作或是说话人的习惯、爱好；而不定式表示具体的、一次性动作.例如：
Smoking is prohibited here.此处禁止吸烟.(抽象动作)
It’s not good for you to smoke too much.你抽那么多烟不太好.
(具体动作)
2.作宾语
1）在动词start,begin,continue,cease等后面,通常情况下不定式和动名词作宾语区别不大.如；
They start taking/to talk about the matter.
他们开始讨论那件事了.
2）在动词like,love,hate,prefer等后面,动名词表示抽象或习惯性的动作,不定式表示具体的、一次性的动作.如：
I like taking a walk after supper.我喜欢晚饭后散步.
It is very hot today.I like to swim after dinner.
今天很热,我想饭后去游泳.
3）有些动词接动名词或不定式时意义完全不同：
●remember,forget,regret后接动名词表示已完成的动作,接不定式表示未完成的动作.如：
You must remember to post the letter.你一定要记得寄信.
I remember posting the letter.我记得已经寄出那封信了.
She forgot to tell her mother about it.她忘了把这件事告诉母亲.
She forgot telling mother about it.她忘了曾告诉过母亲这件事.
I regret to say that I can’t come.我很抱歉,不能来了.
I regret spending so much money.我很后悔花了这么多钱.
●try to do意为“尽力做”,try doing意为“试着做”
Try to be here on time.请尽量按时到这里来.
If the car won’t start,try pushing it.如果这车发动不起来,就试着推一推它.
●stop to do意为“停下来去做另外一件事”,stop doing意为“停下正在做的事”：
He stopped to smoke.他停下来开始吸烟.
He stopped smoking.他戒烟了.
●go on to do意为“接着做不同的事”,go on doing意为“继续做同一件事”：
After writing,he went on to read.写完字后,他接着读书.
He went on working without taking a rest.他一直工作,没有停下来休息过.

it is no good doing sth可以把它理解成一个固定搭配

1.it做形式主语
It is +adj.+(for sb.) +to do sth.
It is time to do/for/that.
2.it做形式宾语
主语+found/think+it +宾补（动词不定式、形容词等）
初中常见的就这几个了,如果有不明白的,欢迎追问、

供参考
这是一个不完整的句子
没有特别的学名的
大概可以说之为elliptical sentence
原句省略了两个连接词 and
至於为何会这样
原因包括下述的可能
(1) 单纯是语法上的错误
(2) 写作技巧
由於只帖一句
没有更多可用作了解作者的笔力/水平的资料
所以难以作出确实的判断
但就以猜想(﹗)来说
较大机会是(2)
因为当考虑到语境
面对著人生的生离死别
内心应当是焦虑
不理智
如果用了工整的句子
信服性就弱了
而且删除那两个没有特别意思的‘ and ’(及其他的一些字)
结构及含义紧密了
可以带来更为震撼的效果
不过
请记住
这只是猜想
出於求个合理的解释
说不定在合理化的过程中
发生了‘想多了’的通病
希望你明白这样的可能性

不定式短语作宾语时,当主句动词是及物动词时,可省略to .但是做真正主语时何时能省略to不记得了.

不定式作主语
1.不定式作主语时,常用it作形式主语,而将作主语的不定式放在句子后部.例如：
It is good to help others.帮助他人是件好事.
It is exciting to surf the Internet.上网是件令人兴奋的事.

11. The article to be completed by me next week is about western cultures.
12. It is very clear that he is not a thief.
13. Whether to leave or to stay depends on your decision.
14. Tall students sit in the back so that all the students can see the blackboard.
15. He was late for the work because of the traffic jam.

是的,可以.这是一个句型,你说得对.It's adj. for sb. to do＋从句

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.

同学,在这个句型中,it是做形式主语的,那真正的主语是to do不定式,有时这个句子中会加入for sb的情况.
比如,举个例子：It is right (for Mary) to finish the homework.it 做形式主语,to finish the homework是真正的主语,为了避免头重脚轻,将其放在后面.

在以it开头的句子后还有一个真正的主语．例如：
It’s difficult for you to catch up with the train in such a short time．
在这个句子中,it是形式主语,you才是真正的主语．翻译：要在这么短的时间内赶上火车是很困难的．
是的

对的.
What you said was true.
What the problem itself is must be obvious.

It is hard (for me) to learn English.这句话里的it就是形式主语,真正的主语是后面的to learn English.因为汉语翻译为：学英语(对我来说)是难的.这种句子用起来就是一开始给别人一种印象(此句是hard),然后别人会注意听是什么难呢?即后面的真正主语就出来了.这种用法使句子的结构会更明了.

it is difficult to learn all day.一天都在学习是件困难的事
it is easy for me to swim in the pool!在游泳池游泳对我来说是件容易的事
it is impolite to speak with the mouth full!当嘴里被塞满（食物）的时候说话时间很没有礼貌的是.

it 在这种句子里可以做实词和虚词 实词it指用于人称和前指（指代刚提到的人或事）,用于无人称是,做主语,表示时间距离天气等.虚词可作形式主语形式宾语,也用于构成的强调句型.
这两句里的it就是用做无人称的,没有具体意义的主语.
eg.It looks as if the college is very small.

逻辑主语：
是非谓语动词动作的执行者或承受者；它们在逻辑上存在着主谓关系或动宾关系,但不能直接作非谓语动词的主语,因此叫逻辑主语.
-----------------------------------
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cost [kɔst]
n.
1.价钱,价格；代价；费用；成本2.牺牲,损失3.支出,花费,开支4.[用复数]【法律】诉讼费(尤指判处败方偿付胜方的诉讼费用)
vt.
1.花费；价格为；值：例句:It must cost a good deal to live here.
住在这里一定要花很多钱的.2.使付出；使损失,使失去：例句:Careless driving cost him his life.
开车不小心使他丧了命.3.需(努力)；费(事)：例句:Courtesy costs little and means much.
礼貌不费事,意义却很大.4.costed,costing 估计…的成本：例句:The work was costed by the engineer at $50,000.
这项工程据工程师估价为50 000美元.
vi.
1.估计成本：例句:We want a man who must be able to cost.
我们需要一个能计算成本的人.2.需要花费,需要付出高昂代价：例句:The furniture in the living room really costs.
起居室里的这套家具真花了不少钱.
短语
1.at all costs不惜任何代价,无论如何2.at any cost=at all costs3.at cost按原价,照成本4.at the cost of以…为代价5.cost someone dear (或 dearly)使某人付出沉重代价,使某人大受损失6.cost what it may不惜任何代价,无论如何7.count the cost(做事前)权衡利害得失；估计不利条件8.to one's cost吃了苦头之后才…,由于付了代价才…
变形:
vt.cost或costedcosting
以上来源于:《21世纪大英汉词典》

it is not sure that wether he would come or not,so he told us do not wait for him.

可数单数用one替,ones替代复数词.
不定冠词后加one,one前一般有定语.
替代若系本身物,it 替单 they 代复.
比较句中替代词,不可数名词that替.
最高级及序数词后,替代词常常被略去.
复数名词替代词,ones 与 those 均用之,
特指 ones 加定冠,those 之前无冠词.
One(s) 的定语多在前,those 的定语必后置.
一、It用作实词
表达以下概念:指代前文提到的事物,前文中的this,that；替代前文中的内容；指代一位性别不明的小孩或未知的人；指代未指明但谈话双方都知道的那件事；指代时间、天气、气候、距离等自然现象.
二、It用作形式主语
替代作主语的从句、动词不定式、动名词,而把真正作主语的从句、动词不定式、动名词置于句尾.
It 作形式主语的常见句型：1.代作主语的动词不定式,其句型为（1）It be adj.(for sb.) to do sth.
（2）It be adj.of sb.to do sth.
（3）It替代作主语的动名词的常见句型：
It’s no good/use doing…
It’s (well)worth doing…
It’s (well)worth one’s while doing/to do It’s (well)worth while doing/ to do
2.It替代作主语的从句常见句型
（1）It is + noun +从句
（2）.It is adj.+clause
It’s surprising that… (should)………竟然……
It’s a pity/shame that…(should)………竟然…
（3）It verb sb.+ clause= It is v-ing + clause
It+surprise/delight/interest/disappoint/worry/disturb/annoy/amaze/bother/concern/frighten/please/anger sb.that…
（4）It verb (to sb.) that…= sb/sth verb to do
(verb = appear,seem,come about,emerge,follow,chance,happen,occur,transpire,turn out ,work out)
（5）It is v-ed that…=sb/sth is to do
(verb=say,report,think,believe,hope,expect,agree,accept,decide,determine,intend,plan,understand,know）
四、It 作形式宾语
用来替代作宾语的从句、动词不定式、动名词,而把真正作宾语的从句、动词不定式、动名词置于句尾.
It 作形式宾语的常见句型：
1.verb+ it+ adj./noun (for/of) to do/clause (verb=think,believe,suppose,consider,feel,make,keep…)
that 用法
pron.
1.那;那个,那人,那事,那东西
That's my English teacher over there.
那边是我的英文老师.
2.(已被提到的)那个,那人,那事,那东西
Who told her that?
那是谁告诉她那件事的?
3.前者
4.(用作关系代词的先行词)那,那个
What is that which you have just stuffed into the plastic cup?
你刚才塞在塑料杯里的是什么东西?
5.(代替句中名词,避免重复)
The output of steel this year has increased by 20% compared with that in 1992.
今年钢产量比一九九二年增长了百分之二十.
6.(用作关系代词,引导关系从句)
a.
1.那,那个
That book isn't mine.
那本书不是我的.
ad.
1.【口】那样,那么
Is the problem that easy?
问题有那么简单吗?
conj.
1.(引导名词从句)
He expressed the hope that we would keep in touch with his firm.
他表示希望我们与他的公司保持联系.
My idea is that we should act on his suggestion.
我的意见是我们应当按照他的建议去办.
That she is still alive is a relief.
她还活着,这是令人感到宽慰的.
2.(引导状语从句,表示原因或理由)因为,由于
We are glad that we have accomplished another arduous task.
我们都感到高兴因为又完成了一项艰巨的任务.
3.(引导状语从句,表示目的或结果)为了,以至于
He is working hard that he can catch up with the class.
他努力用功,以便赶上班上同学.
4.(引导表示愿望,感叹的从句,主句常省略)
That I could stay at home today!
今天要是我能呆在家里多好呀!

不定式、动名词、名词从句作主语时,为了避免头重脚轻,就用代词it等放在前面作形式主语啊,你可以看看这个：http://baike.baidu.com/link?url=ObSOcdfw8Wyc702DSeGHPhhG2l9GkHgIf4czxWO6B7zKD70CLJH7t28K7x0yZ-pb#2

In my opinion, their plane would certainly crash.

1.和他交谈很兴奋（用形式主语）
It's very exciting to chat/talk with him.
2.昨晚你是何时看完书的?
When did you finish reading the book last night?
3.请考虑一下我说的话（what I said）
Please think about what I said.
4.你能告诉我一些有趣的事吗?（用something）
Could/can you tell me something interesting?
5.我们决定执行（carry out）这个计划
I decide to carry out the plan.

It is good for your health to swim .it是形式主语,动词不定式to swim是真正的主语 .其结构为it is +形容词+for sb to do sth
It is bad for us to dance
It is important for them to learn well

当代英语通常用it 做形式主语定式和动名词置于谓语面特别主语较长时或些习惯用法
1） 谓语系表结构（be + 形容词 / 名词）时常定式置构成句型 it + be + 表语（形容词 / 名词）+ 定式
例 1 It is my duty to care for that patient.
照料位病人我职责（做主语定式to care for that patient 置it 形式主语）
例 2 It is not an easy thing to master a foreign language.
掌握门外语件容易事
例 3 It isn’t right to speak ill of someone behind his back.
别人背说坏对
2） 当谓语 takemakepaycost require feelneed 等动词时或者与情绪有关动词 delightamuseexciteannoyirritate 等时习惯上常用 it 做形式主语并定式置
例 1 It takes only ten minutes to get there.
儿只需十分钟
例 2 It pays to be honest.
诚实会吃亏
例 3 It feels good to stay away from the crowded city.
能远离拥挤城市感觉真错
例 4 It annoyed her to see someone spit on the ground.
看见有人随地吐痰让生气
3） 同定式样动名词也用于it + be + 表语 + 动名词句型二者区别参见10．2．2 1）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.
和争辩浪费时间
例 2 A） It is worthwhile to discuss this again.
B） It is worthwhile discussing this again.
点值得讨论下
4） 表语（of）no usenot any usenot the slightest use no good not much goodfun等时常用动名词（用定式）做主语并置用 it 做形式主语
例 It’s no use crying over spilt milk.
覆水难收
5） 动名词还 there be 句型做主语（能用定式）常见形式 there is no + 动名词或 there is no / any + 名词 + 动名词
例 1 There is no denying the fact.
事实无否定
例 2 There isn’t any use trying again.
再试也没用

to be done是不定式的“被动”形式,用于表达将来或即将发生的动作行为.
所以,理论上讲不定式不存在“将来时”这一时态问题,是用法问题.

Walk to the bus stop