That box is too heavy to carry.本句话中没有用it在carry 后,为什么?什么时候it 不能省略.请举例

1. 此句型是一个简单句，句中主语同时还做不定式的宾语，即that box就是carry的宾语，所以carry后面不能再出现it。
此固定句型可以归纳为：sb/sth be + 形容词 + to + 动词原形（+ 介词）。
再如：English is difficult to learn. （English还做learn的宾语）
The man is easy to work with.（The man还做(work) with的宾语）

2. 如果是含有状语从句的主从复合句，则it不能省略，如：
如：That box is so heavy that I can't carry it.
再如：This is such a heavy box that I can't carry it.

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你省略句的知识了解么？ 我给你讲讲吧
1. 省略句的定义 省略是为了避免重复、突出新信息并使上下文紧密连接的一种语法修辞手段。省略在语言中，尤其在对话中，是一种十分普遍的现象。2. 小品词的省略（1）省略介词He spent four hours (in) going over his lessons. 他花了四个小时复习功课。I’ve studied Eng1ish (for) five years.我已学五年英语了。（2）省略连词thatI believe (that) you will succeed． 我相信你们会成功的。It’s a pity (that) he’s leaving. 他要走，真遗憾。I’m sure (that) she will help you. 我肯定她会帮你的。（3）省略关系代词I’ll give you all (that) I have. 我要把我所有的一切都给你。He read the book (which) I got yesterday. 他看过我昨天买的书了。3. 句子成分的省略（1）省略主语Beg your pardon. （我）请你原谅。（Beg前省略了主语I）Take care! 当心！（Take前省略了主语you）Looks as if it will rain. 看起来象要下雨。（Looks前省略了主语it）（2）省略谓语Who next? 该谁了？（Who后面省略了谓语comes）The river was deep and the ice thin. （ice后面省略了was）We’ll do the best we can. 我们将尽力而为。（can后面省略了动词do）（3）省略表语Are you ready? Yes, I am. 你准备好了吗？ 我准备好了。（am后面省略了ready）He was a lover of sports as he had been in his youth. 他还是象年轻时那样，是一位运动爱好者。（had been后面省略了a lover of sports）（4）省略宾语Let’s do the dishes. I’ll wash and you’ll dry. 让我们洗碗吧，我来洗，你来揩干。（wash和dry后面省略了宾语dishes）（5）省略定语He spent part of the money, and the rest he saved. 那钱他花了部分，其余的他都存了起来。（the rest后面省略了定语of the money）（6）省略状语He was not hurt. Strange! 他没有受伤，真奇怪！（Strange前面省略了状语how）　1. 省略句可同时省掉句子几个成份What exciting news! (= What exciting news it is!) 多么令人激动的消息啊！Pity he’s failed. (= It is a pity that he’s failed.) 很遗憾，他失败了。I like him more than her. (= I like him more than I like her.) 我喜欢他更甚于喜欢她。2. 英语中有一些固定的省略结构：a) 在以if, when, though, as if（好象）等连词引导的从句中，如从句中的主要动词是be ，常将主语和动词be 省略。If necessary, we shall send a telegram home. 如有必要，我们就往家里打电报。Whenever possible, he will come to my help. 他一有可能就来帮助我。While cycling, don’t forget the traffic lights. 骑车时，不要忘记看红绿灯。b) 由固定词组引导的疑问句：What about having a game of chess? 下盘棋怎么样？How come they left you alone here? 他们怎么会把你一人留在这里呢？What if it’s raining? 如果天下雨怎么办？Why not try again? 为什么不再试试呢？c) 在口语中，为了避免重复，不定式可以省去和句子前部重复的动词原形而只留下不定式符号to。He may leave if he wishes to. 他可以走，如果他愿意的话。Don’t go till I tell you to. 等我叫你走你再走。3. 被省略的部分一般可以在句子中补上，但有时省略结构已经定型，如果把省略部分补上，反而不合乎习惯。He is taller than I am. 他比我高。（am之后省略tall，补上不合习惯）No parking. 禁止停车。（告示用语 = No parking is allowed here.）有一些习惯表达很难补上所省略的部分：Not at all. 不用谢。No matter. 不要紧。Thanks. 谢谢。　I. 改写句子 使用省略手段避免下面各句重复与冗赘:1. Someone has used my mobile phone, but I don’t know who has used it.2. John worked hard but his brother did not work hard.3. I have lazy students and hardworking students in my class.4. --- Has he ever been abroad? --- No, he has never been abroad.5. If it is necessary, we will finish it ahead of time.6. You must not be late and you must not be absent.7. Give me your name and address, please.8. It is well done.9. Mother can take a rest, Mother should take a rest but Mother won’t take a rest after long hours of work.10.He could answer the questions very well if he would answer the questions but he didn’t answer the questions.省略（ellipsis）是一种为了避免重复、突出新信息并使上下文紧密连接的语法手段。1.简单句中的省略。(1)省略主语（多数属于语境省略）有时，主语和助动词一起省略。(It) Doesn’t matter. 没关系。(I)Beg your pardon. 请原谅。(You)Had a good time？你玩的痛快吗？(I’m)Sorry. 对不起。(Have you)Got any idea？你有什么注意吗？What a beautiful view！多美的景色啊！(2)省略谓语 Who (comes) next? 谁下一个来？(3)省略表语 ---Are you ready? —你准备好了吗？---- Yes, I am —是的，我准备好了。(4)省略宾语 ---Let’s do the dishes. —让我们刷盘子吧。---OK. I’ll wash (dishes). You’ll dry (dishes). —好的。我刷盘子，你把盘子擦干。(5)主(宾)语补足语中的to be常常可省略。We found the situation (to be) serious. 我发觉形势严重。He was thought (to be) the person responsible for the matter. 他被认为是对这件事负责的那个人。(6)在复合宾语结构中省略不定式符号to。不定式作动词make/let/have以及感官动词的宾语补足语时要省略to，但变成被动语态时不能省略。The boss made the workers work all day and all night.老板让工人整日整夜工作。(7)省略介词He spent four hours (in) going over his lessons. 他花了四小时来复习功课。I’ve studied Eng1ish (for) five years. 我学英语五年了。We can stop them (from) moving in. 我们能够阻止他们搬进来。(8)所有格之后的名词如为住宅、商店、工矿、教堂等可以省略。She is going to her uncle’s (house).她要去她的叔叔家。Today I met her at the tailor’s (shop).今天我在裁缝店遇见了她。2.并列句中的省略（通常为承前省略）。通常并列句中后面的分句只要与前边的分句有相同的词语，不管它在句子中作何种成分，都可以省略，反之亦然。His father is a doctor, and his mother (is) a nurse.他的父亲是个医生，母亲是个护士。3.复合句（状语从句、名词性从句、定语从句）中的省略(1)状语从句由(al)though, as if, until, once, unless, when, whether, where, while等引导，且同时具备下列两个条件：1)主句和从句的主语一致，或从句主语为it；2）从句中的主要动词是be的某种形式，则从句中的主语和be动词常可省略。If（you are）in doubt，don’t hesitate to ask me.如果你有疑问，请立刻问我。I did not notice it until (it was) very late.直到很晚，我才注意到它。He won’t come unless (he is) invited.除非他被邀请，否则他不会来。If (it is) possible, I’ll go outing.如果可能的话，我将去郊游。He opened his mouth as if (he were) to say something.他张开嘴好像要说什么。【提醒】比较状语从句中的省略than和as引导的比较状语从句，如果意义明确，than和as后面的任何部分都可省略。I’m taller than he (is tall).我比他高。I know you better than (I know) him.我了解你胜过了解他。This coat is better than that one, but it costs twice as much (as that one). 这件上衣比那件好，但这件是那件价格的两倍。(2)名词性从句1)名词性wh-从句中的省略现象。如果从句谓语与主句谓语相同，从句可省略全部谓语，甚至主语也可省略，只保留一个wh-词。She will go to Beijing, but I don’t know when (she will go to Beijing).她要去北京，但我不知道是什么时候。She can’t come, but I don’t know why (she can’t come).她不能来，但我不知为什么。wh-从句若是被动结构，主谓语也可全部省略，只保留by＋wh词。The cup was broken by someone, but I wonder (the cup was broken) by whom．这个杯子被某人打碎了，但我纳闷是被谁打碎的。如果从句相同而wh-词不同，则可省略第一个从句而把两个wh-词连接起来。I don’t know when (to meet him) and where to meet him.我不清楚什么时间、什么地点见他。2)在与表示命令、愿意、建议等相关的名词性从句中，常用should型虚拟，而从句谓语部分中的should常省略。We require that he (should) tell us the truth.我们要求他告诉我们真相。3)在句型“It is necessary/surprising/suggested/natural /strange…+ that从句”中，从句通常用should型虚拟，而should常常可以省略。Everyone thinks it is necessary that our research group（should）begin the experiment at once．每个人都认为我们的研究小组立刻开始实验是必要的。4)在宾语从句中，连词that常可省略。但如果有两个宾语从句，第二个宾语从句的that不能省；如果宾语从句和主句谓语之间有插入语时，that也不能省略。I believe (that) you will succeed.我认为你会成功