作者：刘俊雪    文章来源：本站征文来稿    点击数：    更新时间：6/5/2007

一、表示“能力”指自身具有的能力

can与be able to

(1)   在现在时中通用。He can swim. = He is able to swim.

(2)   在过去时中，否定式两者通用：

--- Because he was young , he couldn’t(wasn’t able to) swim across the river.

但在肯定句中两者表达的意思不同：

   could 指本身具有的能力They could read and write.他们能写会读。（指自身具有的能力。）

而was (were) able to 指不但自身具有能力而且通过主观努力而能够做成某事：

I was able to swim across the river when I was only ten years old.十岁时，我就能设法游过这条河。

He was able to flee Europe before the war broke out.战争爆发前，他能够设法逃离了欧洲。

（NMET97）The fire spread through the hotel very quickly, but everyone ____get out.   A. had to  B. would C. was able to  D. could  (答案C)

二、表示委婉地“征询对方意见”或“提出请求”（1.  can\could  2.  may\might

 3.  shall  4 . will\would ）

(1) can (could更委婉些)指客观条件允许： “能----吗？”意思是：我这样做，你是否方便。

--- Can\Could you lend me some money? 你可以借给我点钱吗？（客观条件是：你有钱的情况下）

--- Can\Could I have the TV on ? 我可以开着电视吗？（客观条件是：在不打搅你的情况下。）

--- Yes, you can . ( No, you can’t) 答语不用could\couldn’t.

(2)   may (might更委婉些)指主观请求允许：“可以----吗？” 意思是：我想这样做，你是否允许。

--- May I come in ?  Yes, you may.  (No, you can’t) 

--- May\ Might I ask you a question?  Sure, you may (No, you’d better not.)

(3)   shall 用于第一、三人称：“我们---好吗？要----吗？”表示与说话对象进行商议。

--- Shall we go to see a film after school? 

--- Shall I show him in ? 

--- Shall I get some chalk for you ?  

--- Shall he wait for you at the school gate ?  

--- Let’s have a rest , shall we?

(4) will\ won’t (would 更委婉些) 指意愿，常用于第二人称“您做----好吗？您愿意-----吗？”

--- Will \Won’t\Would you sit down?您请坐好吗？

--- Will \Would you please do it for me ? ---- Of course, I will.  

--- Would you like some tea?

--- Would you like to help me with my English study ? Yes, I’d like to.

三、表示推测：(1. must  2. can  3. could  4 .would  5. should  6. may\might )

（1）must （只用于肯定句）根据具体事实所做的很有把握的主观肯定判断和猜测：

 “一定----，肯定---”：

--- You have worked hard all day, you must be tired. （对现在事实的判断）

--- It’s dark. Mother must be waiting for my return.

--- I didn’t hear the phone. I must have been asleep. （对过去事实的判断）

--- It must have rained last night, for the road is wet.

注意：must 的疑问式、否定式和反意疑问句的用法。

（a）must 对现在事实作判定市的疑问式、否定式应用 can \can’t(couldn’t语气稍缓和)代替，表示“惊异、猜测、怀疑、不相信” 意思为“可能----吗？、（绝）不可能-----.”

--- Li ping .someone is waiting for you at the school gate.

--- Who can it be ?（能是谁呢？表猜测）

--- I heard they went skiing in the mountain last winter.

--- It couldn’t be true because there was little snow there.(这事不可能是真的，因为那里就没有雪。这里是现在对此事可能性的否定判断可以直接说It can’t be true，不是对过去的他们去滑雪行为的判断，那样须用They couldn’t have. (gone skiing).

--- Mr. King is on time for everything .How ___it be that he was late for the opening ceremony?（金先生什么事都准时的，这个开幕式迟他到怎马可能呢？）

A.     can  B. may  C. should  D. must (答案A. 表惊讶、怀疑、不相信)

而对过去的事实进行疑问、否定猜测时，应用can\could have done

                                     Can’t \couldn’t have done

--- How can Tom know this? Can\Could Mary have told him?

--- My English-Chinese dictionary has disappeared. Who could have taken it?

--- The ground is dry. It can’t\couldn’t have rained last night.  

--- Do you remember the day when we went to the Great Wall?

--- I can’t remember it well, but _______some time last autumn?

A. might it be B. could it have been C. could it be D. must it have been (答案B.)

(这里是对the day was some time last autumn这一过去事情的疑问推测.不选C)

--- From what we learn about their hotel, the service and the weather, they can’t \couldn’t have enjoyed their holiday very much.

(b) 反意疑问句的助动词应按主句中must 后实意动词情况构成疑问。

--- He must be 40 years old now, isn’t he?

--- Your wife must be waiting for you at home, isn’t she?

--- He must have told her about it, hasn’t he ?

--- It must have rained last night, didn’t it ？

--- You must have studied English before, haven’t you?

 (c)  can 不能用于肯定句中对具体事实进行推测。用在肯定句中表推测的can是下列情况。

(2) can 表抽象可能性，指只是理论上的可能性，不表示事情发生的实际可能性或真实性。用于肯定、否定、疑问都行。

 --- Mr. Smith is in poor health. He can be ill at any time. 史密斯先生身体欠佳，他可能随时生病。（指从逻辑上讲一般情况下，他有随时生病的可能性，这是对他身体状况的客观现实做出的概括性的、抽象性的推论，并不是说他现在可能生病了。

试比较：Mr. Smith looks pale. He may be ill.（史密斯先生脸色苍白，他也许生病了。根据他脸色苍白的具体事实推测，推测他此刻也许是生病了）

 --- Going on a trip can be expensive and difficult, so making a plan is necessary. 去履行可能既昂贵又困难，所以做个计划是必要的。（指按常理和客观现实，一般情况下，履行可能是昂贵有困难的。这是一个泛泛的推论，并不指具体谁的或去哪个地方的旅行。）

--- Facts prove that the world’s economic development is not a win –lose game but one in which all can be winners.事实证明世界积极的经济发展不是一场或输或赢的游戏，而是在其中大家都可能是赢家的游戏。（按客观存在的现实，一般情况下，谁都有赢的可能性，这是一个抽象的理论。）

--- You can’t park on the pavement.你绝不可能在人行道上停车。

(是按常规，从理论上概括出的一般情况下的一个宏观的理论：没有在人行道上停车的可能性，并不是你现在要在人行道上停车而行人太多，客观条件不允许的具体情况。试比较下列句子。)

--- It was crowded on the street and a driver wanted to stop his car on the pavement, there wasn’t a place for him to do that. His partner said:“You can’t park on this such crowded pavement.”

(这里指具体情况下客观条件不允许)

--- Can a person never die?(人能永远不死吗？不针对具体的某个人)

--- A person can’t never die.(一个人不可能永远不死。（一个泛泛的理论，不指具体的某个人）

词组：1、can not\never-----too\enough表示“无论怎么----也不过分，越----越好”

--- You can not\never be too careful\careful enough.你越小心越好。

--- You cannot remember enough English words.你记的英语单词越多越好

  2、cannot but do sth 表示“不得不，只好”

   --- I could not but choose to go.(我只好去)

   --- I cannot but admire your bravery.（我不得不钦佩你的勇气）

3、can’t help doing 禁不住

--- We cannot help thinking that he is still alive.

我们禁不住认为他还活着。

--- He cannot help having big ears.他禁不住打听别人的事。

4、can’t help to do不能帮着做----

--- He couldn’t help to do any housework.他不能帮着做任何家务。

 (3)could 表具体可能性，指按客观现实逻辑地概括出一般情况下某事要发生实际可能性或可能出现的情况。（用在肯定句、条件句中）

   --- My wife is in hospital now. Our baby could arrive at any time.我妻子进产院了，我们的孩子可能随时出生。（按常理和客观现实，一般情况下，不出意外的话，有孩子可能随时出生的真实性。）

--- I’m so unhappy I could weep.(我心里很不痛快，我都要哭了)

--- What’s for dinner? I could eat a house.(晚饭吃什么？我饿极了)

--- We could write a letter to the headmaster.(实际上，我们可以给校长写封信)

--- You could try his home number.(你可以试着打他家的电话)

--- Officials said costs for cleaning up could reach US$188,000. The UN has urged

other countries to help.(指按客观情况推断，费用高达那么多是实际情况)

   --- Don't worry, they could have just forgotten to phone.别着急，他们可能仅仅是忘了打电话。（意指：他们很可能是真的忘了打个电话。不会出事的）

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