It 作形式主语的三种句型
It作形式主语
It作形式主语和形式宾语，虽然是我们高中就学过的语法点，但在考研英语题型中，无论是在阅读还是翻译，常会考查该知识点。
一.it作形式主语。
英语中不定式、动名词、主语从句在作主语时主语往往显得很长，使整个句子看上去有点头重脚轻，这样就借助“it”来作形式上的主语，而把真正的主语移到句子后边去，使全句结构上前后平衡。
“it”并无实际意义。
It之后的谓语部分，可以是be+形容词，be+名词(词组)，或be+过去分词等。
it 作形式主语，主要分为下列三种情况：不定式作主语、动名词作主语和名词从句作主语。
具体分析如下：一、不定式作真正主语主要用于下列句型：1. It + is/was + adj./n.+ to do sth.A.名词作表语。
主要有a pity, a pleasure, an idea等。
It is a good idea to think this way. 这样考虑问题是个好办法除be外，其他动词也有跟名词的情况。
例如：It seems a pity to waste the food. 浪费这些食物真可惜。
B. 形容词或形容词短语作表语。
分两种情况：1.下列形容词；kind,good,nice,clever,wrong,right,foolish,wise,unwise,stupid,rude,careless,cruel,brave ,naughty,polite,selfish等词作表语时，不定式前常可加一个由of引起的短语，来说明不定式指的是谁的情况。
形容词表示不定式逻辑主语的特征，即形容词用来描述不定式行为者的性格、品行等。
例如：It was foolish of you to leave school.你中途退学，真是傻瓜。
2. 其他形容词作表语，常见的形容词有：necessary, hard，important, difficult, easy, possible，common等。
“it”做形式主语
二、It 作形式宾语
We think it important to learn a foreign language.
常用的动词：think,believe,make,find,consider,feel； We think it our duty to clean our classroom every day. He felt it important learning English well.
1. 对我们来说完成这项工作很难。 It is difficult for us to finish the work. 2. 对孩子们来说横穿马路很危险。 It is dangerous for children to cross the street. 3. 对我们来说保护环境是必要的。 It is necessary for us to protect the environment.
• It is kind of you to help me. = You are kind to help me.
1. 他丢了太多东西，真是太粗心了。
It was careless of him to lose so many things.
2. 节约用水是很明智的。 It is wise to save water.
They found it difficult that they would finish their work
in two days.
4. It 作形式宾语. We think it important to learn a foreign language. 该句型中的it 作形式宾语，为了记忆方便我们可称该句型 为"6123结构"。 6指主句中常用的动词： think,believe,make,find,consider,feel； 1指的是形式宾语it； 2指的是宾补的两种形式：形容词或名词； 3指的是真正宾语的三种形式：不定式短语，动名词短语 或that引导的宾语从句。 We think it our duty to clean our classroom every day. He felt it important learning English well. They found it difficult that they would finish their work in two days.
雅思考试it作形式主语的用法
雅思考试it作形式主语的用法雅思考试中，it这个词的用法还是很变化多端的。
考虑到它在雅思考试中的高出现率，今天天道雅思论坛小编就来为同学们做一个雅思辅导，说一说it的用法。
以下是今天的精彩讲解。
雅思考试，先来了解一下it作形式主语的用法：(一)什么是形式主语作为形式主语的it并无实际语义，是为满足语法上的需要，避免句子头重脚轻，它代替的是句子的逻辑主语。
(二)形式主语it所在的句型类别1、作不定式的形式主语(1)句型It;is;+;a;pity/a;pleasure;+;to;do;sth.It;is;+;wrong/right/wise/cruel/naughty/selfish;+;of;sb.;+;to;do;sth.It;is;+;necessary/hard/important/difficult/possible/common;+;for;sb.;+;to;do;sth.(2)雅思范文写作实例A.;Beyond;this,;it;is;also;of;paramount;importance;to;use;theaters;and;museums;as;the;“social;classroom”;of;culture;and;history;education.B.;I;think;it;is;by;no;means;pointless,;in;any;way,;to;try;to;keep;traditions;alive;wit h;technology.;(Cambridge;3,;P162,;Task;2)C.;It;is;difficult;to;say;who;has;the;right;to;judge;whether;children;working;is;“wron g”;or;“valuable”.;(Cambridge;3,;P164,;Task;2)D.;…,;it;is;important;to;remember;that;children;need;to;d evelop;skills;other;than;inte llectual;ones,;and;…(Cambridge;3,;P168,;Task;2)E.;Finally,;I;think;that;it;is;also;important;to;remember;that;children;need;to;relax;a s;well;as;work,;…(Cambridge;3,;P168,;Task;2)F.;Because;people;interpret;happiness;for;themselves;in;so;many;different;ways,;it;is;d ifficult;to;give;ang;definition;that;is;true;for;everyone.;(Cambridge;4,;P165,;Task;2)G.;It;is;quite;common;these;days;for;young;people;in;many;countries;to;have;a;break;for ;studying;after;graduation;from;high;school.;(Cambridge;5,;P165,;Task;2)H.;They;may;think;that;it;is;better;to;continue;in;a;particular;job,;or;to;do;sth;compl etely;different;from;from;a;university;course.;(Cambridge;5,;P165,;Task;2)据本人统计，剑桥官方的雅思真题(剑3—剑8)几乎每一篇Task2考官范文中，都有所使用，甚至在有的文章中出现三次之多。
it作形式主语的特殊句型
昂立外语：/
it作形式主语的特殊句型
it用作形式主语有不少非常有用的句式，比如以下一些就值得注意：
一、用于it seems (appears) that…句式。
如：
It seems that he knows everything. 他好像什么都知道。
It appears that we may be mistaken. 看来我们可能弄错了。
但是，英语一般不说it looks that…，可改用it looks as if…。
如：
It looks as if we’ll be late. 我们似乎要迟到了。
二、用于it happens (occurs) that…句式。
如：
It happened that I was standing next to him when he collapsed. 他倒下的时候我恰巧站在他旁边。
It didn’t occur to him that she would refuse his invitation. 他没有想到她会拒绝他的邀请。
三、用于it follows that…句式。
如：
He is wrong, but it does not follow that you are right. 他错了，但并不能由此推断你是对的。
She’s not in the office but it doesn’t necessarily follow that she’s ill. 她不在办公室, 并不见得就是病了。
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it做形式主语
it 作形式主语的用法代词it可以用来代替一个名词、一个短语、一个句子，在句中作主语或宾语。
1. it作主语指时间、日期、天气、距离和气温等。
It’s nine o’clock now. （指时间）It is much colder than it was yesterday. （指天气）It is about 5 kilometres from here to my home. （指距离）It is very hot in the classroom. （指气温）It is Sunday today. （指日期）2. It作形式主语代替动词不定式，常用于以下句型中：(1) it is/was adj. (of/for) sb. to do sth.当表语是与事物的特征有关的形容词时，如easy, difficult, hard, important, necessary等，用介词for.It is difficult for Lucy to learn physics.It is interesting for us to play badminton.当表语是描述人的性格、品质的形容词时，如kind, nice, polite, clever, foolish, stupid等，用介词of.It is kind of you to help me.It is clever of the little child to work out such a difficult math problem.(2) it takes/took sb. some time to do sth.It takes him an hour to do his homework every day.(3) it is/was n. to do sth.It is great fun to have a party here.It was a great pleasure to work with you.3.it作形式主语还常用于句型“it seems/appears that +从句”，表示“似乎/好像/看来......”It seems that he knows everything.It appears that we are wrong.。
it作为形式主语
形式主语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 +be+(the)+adj+for sb+to do sth.形式主语it作为形式主语的it并无实际语义，只是为满足语法上的需要，避免句子头重脚轻，它代替的是句子的逻辑主语，概括起来会有如下情况：1. 句子的逻辑主语为不定式如：It is very kind of you to give me a hand in time.It is my pleasure to address the meeting.It was pretty hard for him to bring up the child on his own.2. 句子的逻辑主语为从句如：It so happened that the tickets were sold out.It is extremely obvious that she has been lying about her identity.It suddenly occurred to me that the message must have been hiddenbetween the layers of the briefcase.3. 句子的逻辑主语为动名词短语这类句子远不如前面两种出现率高，主要在（no good, waste, useless, no use,dangerous）等词语的后面用如：It is no use reasoning with him.It is no good reading in dim light.作形式主语的代词只能用it ，不能用that，this等词。
例如:1) It is easier to lose friends than to make friends. 交朋友难而失去朋友比较容易。
it作形式主语的句型
it作形式主语的句型
1、It + be + 形容词 + that从句：表示某事物具有某个特征或属性。
例如：It is important that we arrive on time.（我们按时到达很重要。
）
2、It + be + 名词短语 + that从句：表示某个事物符合某个特征或属性。
例如：It is a shame that he didn’t come to the party.（他没来参加聚会真是遗憾。
）
3、It + be + 过去分词 + that从句：表示某个动作已经被完成或已经发生。
例如：It is said that the book will be published next month.（据说这本书将在下个月出版。
）
4、It + be + 现在分词 + that从句：表示某个动作正在进行或正在发生。
例如：It is raining hard outside.（外面正下着大雨。
）
5、It + be + 不定式短语 + that从句：表示某个动作将要发生或被计划。
例如：It is to be announced tomorrow if the meeting will be held as scheduled.（明天将宣布会议是否按计划举行。
）
这些句型中的“it”通常被称为“形式主语”，因为它在句子中充当主语，但实际上并不是真正的主语。
这种用法是为了使句子更加通顺或强调某种特定的信息。
it作形式主语和形式宾语
it既可作人称代词，用来表示动物或无生命的物体等，也可以作无人称代词，在句中作主语，表示“时间”、“距离”、“天气”等。
它还可以引导强调句，使语气得以加强。
此外，it在句中能作引导词，充当句子中的形式上的成分，如形式主语、形式宾语等。
㈠it作形式主语：it作形式主语时没有具体的意义，而只是帮助把真正的主语移到句子后部去，使句子显得平稳一些。
it作形式主语时，可以代替三种形式：不定式、动名词和主语从句。
⒈it代替不定式短语常用于下列句型中：It + be +形容词+ (for/of sb.) to do sth.It + be + 名词+ to do sth.It takes sb. some time to do sth. 干…花了某人…时间It’s up to sb. to do sth. 干…是某人的职责或义务如：①It is everyone’s duty to obey the law.遵守法律是每个人的义务。
（it作形式主语, 代替不定式短语to obey the law）②It is difficult for a foreigner to learn Chinese.对于一个外国人来说，学习汉语是困难的。
（it作形式主语, 代替不定式短语to learn Chinese）③It is not right to use these places as rubbish dumps.把这些地方用来倒垃圾是不对的。
（it作形式主语, 代替不定式短语to use these places as rubbish dumps）④It took them a year to build the bridge. 建这座桥花了他们一年的时间。
（it作形式主语, 代替不定式短语to build the bridge）⑤It is bad manners to stare at people.瞪着眼睛看人是不礼貌的。
常用it作形式主语的句型结构
常用it作形式主语的句型结构◇It + be +形容词+ that-从句可用于此句型的形容词有: wonderful,true，good，right，wrong，（un）important，useless，surprising，clear，unusual，lucky，certain, necessary等。
如：①It is quite certain that he will be at the meeting.②It is surprising that he came late to school this morning.③It is important that we(should)study hard注意: 该句型中的形容词是（un）important, necessary等时，从句应为(should )+动词原形。
◇It + be + -ed 分词+ that-从句适用该句型的过去分词主要有: said,reported,announced,hoped,thought，told，believed，expected，decided，suggested,known等。
如：①It is hoped that one day they will have enough animals to set them free②It is well known that the earth goes around the sun.③It is suggested that the sports meeting (should ) be put off until next week. 注意: 该句型的过去分词是suggested时，从句应为(should )+动词原形。
◇It + be +名词+ that-从句适用该句型的名词（词组）有: a pity，an honor，a good thing，a fact，a surprise，good news, one’s duty等。
“It”作形式主语和形式宾语的用法详解
it作形式主语和形式宾语一、it作形式主语：当不定式（短语）、动名词（短语）或从句在某个句子中作主语时，为保持句子结构前后平衡，避免头重脚轻，因此常用it作形式主语置于句首，而将真正的主语放在句尾。
此时it只起先行引导作用，本身无词义。
it作形式主语时，可以代替三种形式：不定式、动名词和主语从句。
1. it代替不定式短语常用于下列句型中：如：It is everyone’s duty to obey the law.遵守法律是每个人的义务。
It is difficult for a foreigner to learn Chinese.对于一个外国人来说，学习汉语是困难的。
It took them a year to build the bridge.建这座桥花了他们一年的时间。
It is bad manners to stare at people.瞪着眼睛看人是不礼貌的。
It is up to us to help those in need.帮助那些有困难的人是我们的责任。
eg. It’s important for us to learn English well.学好英语对我们来说是重要的。
It’s kind of you to help me.感谢你对我的帮助。
⒉ it代替动名词短语：It’s no good/use/useless/a waste of time doing sth.做某事没有用/浪费时间①It is no good/ use having a car if you can’t drive.如果你不会开车，有车也没用。
②It is a waste of time watching TV.看电视是浪费时间。
（it作形式主语,代替动名词短语watching TV）⒊ it代替主语从句：这个主语从句可以用连词that引导，也可以用连接代词或连接副词引导。
如：①It was clear that they had no desire for peace.很清楚他们没有和平的诚意。
it做形式主语的个句型
i t做形式主语的个句型集团档案编码：[YTTR-YTPT28-YTNTL98-UYTYNN08]1.Itis/was+被强调部分+that/who...该句型是强调句型。
被强调部分可以是主语，宾语，表语或状语。
强调的主语如果是人，that可以由who换用。
如果把这种句型结构划掉后，应该是一个完整无缺的句子。
这也是判断强调句型与其它从句的方法。
2.Itisnotuntil+被强调部分+that...该句型也是强调句型。
主要用于强凋时间状语，译成汉语“直到……才……”，可以说是not...until...的强调形式。
Itwasnotuntilshetookoffherdarkglasses thatIrealizedshewasafamousfilmstar. =NotuntilshetookoffherdarkglassesdidI realizeshewasafamousfilmstar.=Ididn'trealizeshewasafamousfilmstaru ntilshetookoffherdarkglasses.3.Itisclear/obvious/true/possible/cer tain…that…该句型中it是形式主语，真正的主语是that引导的主语从句，常译为“清楚（显然，真的，肯定……）”是主语从句最常见的一种结构。
Itisveryclearthathe'sroundandtalllike atree.=Thathe'sroundandtalllikeatreeisveryc lear.4.Itisimportant/necessary/right/stran ge/natural...that...+(should)do该句型和上一个同属一个句型。
由于主句中的形容词不同，that后的从句中要用虚拟语气（should+动词原形），should可以省去。
It句型归纳最全总结
It 句型归纳总结I．It 作形式主语的句型①It + be + adj (kind , wise , careless, clever…) + of sb to do sthEg: It is wise of him to make such a decision.他做出这样的决定是明智的。
②It + be + adj(easy , safe , common , normal …) + for sb to do sth该句型中adj若为important, necessary, natural, possible, strange, vital, essential等可改为从句,从句用should+动词原形。
.Eg: It’s very dangerous for children to play with fire.It is important for us to learn English well. (= It is important that we (should) learn English well.)③It + be +V-ed( reported, believed, said, suggeste d…) + that 从句该句型中：⑴若V-ed为reported, believed, said等常译为“据报道”，“据认为”，“据说”。
⑵若V-ed为suggested, ordered, demanded, insisted等表示“建议”,“命令”的词，从句要用虚拟语气(should)+动词原形。
Eg: It is suggested that the book (should) be revised.It’s said that Tom has been back from abroad. (=Tom is said to have been back from abroad.)④It + be + n (a pity, a shame, a fact , good new s…) + that 从句:该句型中，从句一般用(should) + 动词原形，表示出乎意料，常译为＂竟然＂。
it作形式主语的几大基本句型
it作形式主语的几大基本句型形式主语，顾名思义，即形式上是主语，但实际上不是真正的主语，真正的主语是后面的动词不定式、动名词或主语从句等。
如果让后面的动词不定式、动名词或主语从句用作主语，那么在句式上容易造成“头重脚轻”，句式不平衡，故往往用it作形式主语，而把真正的主语放在后面。
it在这里没有任何意义。
it作为形式主语常常用在以下几个句型。
◇It’s +形容词/名词+that +从句It is necessary that you train yourself before the walk.It is important that you have your own support team.It was a pity that he lost the game.It’s a pity that you missed the exciting football match.◇It’s +形容词+(for sb) +to do sthIt is difficult to walk through eight country parks.It is useful to have support teams.It is important for him to finish it in ten hours.It is necessary for people to learn team spirit.特别提醒：☆该句型的否定结构为It’s +形容词+(for sb) +not +to do sth It is very important not to give them too much food.It is very polite not to speak loudly in public.☆该句型有时可以与It is + adj. + that从句的句型互换使用，意思不变。
如：It is necessary for you to train yourself before the walk. = It is necessary that you will train yourself before the walk.◇It’s +名词+to do sthIt’s everyone’s duty to obey the law.It is not a good habit to stay up too late.As a student, it is bad manners to come late to class.It is my turn to clean the blackboard.◇It takes sb +some time to do sthIt took us two hours to set up the stage.◇It seems that +从句It seemed that he didn’t tell the truth.◇It’s +过去分词+从句惯用句型有：It is said that(据说), It is reported that(据报道), It is believed that(大家相信), It is hoped that(大家希望), It is well known that(众所周知), It is thought that(大家认为), It is suggested that(据建议)等。
It作形式主语的几大基本句型
“I t”作形式主语的几年夜基本句型之迟辟智美创作2011-06-07 22:27:53| 分类：语法归纳 | 标签： |字号年夜中小定阅◇It’s +形容词/名词+that +从句It is necessary that you train yourself before the walk.It is important that you have your own support team.It was a pity that he lost the game.It’s a pi ty that you missed the exciting football match.◇It’s +形容词+(for sb) +to do sthIt is difficult to walk through eight country parks.It is useful to have support teams.It is important for him to finish it in ten hours.It is necessary for people to learn team spirit.特别提醒：☆该句型的否定结构为It’s +形容词+(for sb) +not +to dosthIt is very important not to give them too much food. (7B 100) It is very polite not to speak loudly in public.☆该句型有时可以与It is + adj. + that从句的句型互换使用，意思不变.如：It is necessary for you to train yourself before the walk. = It is necessary that you will train yourself before the walk.◇It’s +名词+to do sthIt’s everyone’s duty to obey the law.It is not a good habit to stay up too late.As a student, it is bad manners to come late to class.It is my turn to clean the blackboard.◇It takes sb +some time to do sthIt took us two hours to set up the stage.◇It seems that +从句It seemed that he didn’t tell the truth.◇It’s +过去分词+从句惯用句型有：It is said that(据说), It is reported that(据报道), It is believed that(年夜家相信), It is hopedthat(年夜家希望), It is well known that(众所周知), It is thought that(年夜家认为), It is suggested that(据建议)等.It is reported that 16 people were killed in the earthquake.。
常用it作形式主语地句型结构
常用it作形式主语的句型结构◇It + be +形容词+ that-从句可用于此句型的形容词有: wonderful,true，good，right，wrong，（un）important，useless，surprising，clear，unusual，lucky，certain, necessary等。
如：①It is quite certain that he will be at the meeting.②It is surprising that he came late to school this morning.③It is important that we(should)study hard注意: 该句型中的形容词是（un）important, necessary等时，从句应为(should )+动词原形。
◇It + be + -ed 分词+ that-从句适用该句型的过去分词主要有: said,reported,announced,hoped,thought，told，believed，expected，decided，suggested,known等。
如：①It is hoped that one day they will have enough animals to set them f ree②It is well known that the earth goes around the sun.③It is suggested that the sports meeting (should ) be put off until next week.注意: 该句型的过去分词是suggested时，从句应为(should )+动词原形。
◇It + be +名词+ that-从句适用该句型的名词（词组）有: a pity，an honor，a good thing，a fact，a surprise，good news, one’s duty等。
it 用作形式主语的句型
4) It + be +过去分词+从句 It is said that... 据说…… It is known to all that... 众所周知…… It is reported that... 据报道…… It is believed that...据相信……;人们相信…… It is suggested that + (should）do... 建议…… *_________________(据报道)20 people were killed in the accident. *__________________(建议)we should eat more vegetable and do more exercise.
1. 好像是我错了。 It appears that I am wrong. 2. 碰巧他得去参加一个会议。 It happened that he had to attend a meeting. 3. 我突然想起他会说英语。 It occurs to me that he can speak English. 4. 原来他是Mary的朋友。 It turned out that he was Mary’s friend.
1. 令人吃惊的是他改变了主意。 It is a surprise that he changed his mind. 2. 真可惜你错过了这么一个好机会。 It is a pity that you missed such a good chance. 3. 事实是我们周围的污染正变得越来越严重。 It is a fact that the pollution around us is becoming more and more serious.
It构成的句型小结
It构成的句型小结一、It作形式主语的句型为使句子平衡，往往把it置于句首作形式主语，而把真正的主语（通常是不定式短语、动名词短语或主语从句）移至句末。
It 作形式主语的句型有：㈠it 代指不定式短语的句型1. It takes + (sb.) + sometime / some money + to do sth.做某事花去某人多长时间Just a minute, it won't take me long to change. 等一下,我很快就可以换好衣服。
It takes five minutes to walk there. 走到那里需要5分钟。
It took me three days to prepare for the examination. 我花了三天准备那次考试。
It takes a lot of money to buy a house like that. 买那样一幢房子需要很多钱。
2.It takes + 名词+ (for sb.) to do sth. 某人做某事需要… …It took great imagination as well as patience for Annie to teach me to speak.对安妮来说，教我说话需要极大的信心和想象力。
It takes time for them to prepare for the great changes that take place in space.他们需要很多时间准备应付太空中所发生的巨大变化。
3.It is + 形容词+ (for / of sb.) to do sth. 某人做某事是… …的for sb. 的句型常用于表示事物的特征特点，表示客观形式的形容词，如easy, difficult, hard, important, possible, impossible, comfortable, necessary, better等。
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It 作形式主语的三种句型
【热身训练】
Ⅰ. 单项选择
( ) 1. — Is it to talk with her?
— Yes. She is not always friendly to others.
A. kind
B. easily
C. meaningful
D. tough
( ) 2. It is meaningful us action to protect our Earth.
A. of; to take
B. for; to take
C. of; taking
D. for; taking
Ⅱ. 按要求完成下列各题。
(第3~4题翻译句子；第5题改写同义句)
3. 加入他们做一些志愿者工作很有趣。
4. 对孩子们来说，在马路上踢足球是危险的。
5. It is important that we should understand our parents.
is important us our parents.
【融会贯通】
It is + adj. + that从句
用法简介
该句型中that从句在句中作主语，it作形式主语。
这个句型常用来说明对某事的感受。
句型意为“某人做某事是……”。
典型例句
It’s necessary that you wash your hands before meals. 你饭前洗手是必要的。
It’s + adj. + (not) to do sth.
用法简介
这是动词不定式短语作主语的最常用句型。
动词不定式短语作主语时，常用it 作形式主语，而将真正的主语(动词不定式短语)后置，以保持句子的平衡。
句型意为“(不)做某事是……”。
典型例句
It’s important to learn a foreign language. 学一门外语是重要的。
It is very polite not to speak loudly in public.
在公共场合不大声说话是非常礼貌的。
It is + adj. + for / of sb. (not) to do sth.
用法简介
这是I t’s + adj. + (not) to do sth.的延伸句型。
当动词不定式短语作主语，且其含有自己的逻辑主语时，在动词不定式短语之前用介词for或of引出其逻辑主语。
典型例句
It is very easy for me to finish the work. 完成这项工作对我来说是很容易的。
It’s very kind of you to help me. 你能帮我，真好。
【温馨提示】
1. It is + adj. + for sb. (not) to do sth. 句型有时可以与It is + adj. + that从句的句型互换使用，意思不变。
如：
It is necessary for you to train yourself before the walk.
= It is necessary that you will train yourself before the walk.
2. 在第三种句型中，若形容词是描述动词不定式行为者的性格、品质的，如kind,
clever, careless等，动词不定式短语之前用of sb。
如：
It’s clever of you t o answer the question correctly.
你真聪明，正确回答了这个问题。
若形容词仅仅是描述事物，不对动词不定式行为者的品格进行评价，动词不定式短语之前用for sb。
这种形容词有difficult, easy, hard, important, dangerous等。
Key：
1-2 DB
3. It is interesting to join them in doing some voluntary work.
4. It is dangerous for children to play football on the road.
5. It; for; to understand。