should 和could的用法与区别

题目

should 和could的用法与区别

答案

情态动词是一种本身有一定的词义,但要与动词原形及其被动语态一起使用,给谓语动词增添情态色彩,表示说话人对有关行为或事物的态度和看法,认为其可能、应该或必要等.
情态动词后面加动词原形.
情态动词有四类：
①只做情态动词：must,can(could),may(might),ought to
②可做情态动词又可做实义动词：need,dare
③可做情态动词又可做助动词：shall(should),will(would)
④具有情态动词特征：have(had,has) to,used to
特点:
情态动词无人称和数的变化, 情态动词后面跟的动词需用原形,否定式构成是在情态动词后面加 "not". 个别情态动词有现在式和过去式两种形式, 过去式用来表达更加客气, 委婉的语气, 时态性不强, 可用于过去,现在或将来.情态动词属非及物动词,故没有被动语态.
He could be here soon.
他很快就来.
We can't carry the heavy box.
我们搬不动那箱子.
I'm sorry I can't help you.
对不起,我帮不上你.
基本助动词与情态助动词最主要的区别之一是,基本助动词本身没有词义,而情态助动词则有自己的词义,能表示说话人对有关动作或状态的看法,或表示主观设想：
What have you been doing since? （构成完成进行体,本身无词义）
I am afraid I must be going. （一定要）
You may have read some account of the matter. （或许已经）
除此之外,情态助动词还有如下词法和句法特征：
1） 除ought和used以外,其他情态动词后面只能接不带to的不定式.如果我们把ought to和used to看做是固定词组的话,那么,所有情态动词无一例外地只能接不带to的不定式：
We used to grow beautiful roses.
I asked if he would come and repair my television set.
2） 情态助动词在限定动词词组总是位居第一：
They need not have been punished so severely.
3） 情态助动词用于第三人称单数现在时的时候,没有词形变化,即其词尾无-s形式：
She dare not say what she thinks.
4） 情态动词没有非限定形式,即没有不定式和分词形式,也没有相应的动名词：
Still, she needn't have run away.
5） 情态助动词的“时”的形式并不是时间区别的主要标志.在不少场合,情态助动词的现在时和过去时形式都可以表示现在、过去或将来时间：
Would you mind very much if I ask you to do something?
She told him he ought not to have done it.
6） 情态助动词之间是相互排斥的,即在一个限定动词词组中只能出现一个情态助动词,但有时却可以与have和be基本助动词连用：
You should have washed the wound.
Well, you shouldn't be reading a novel.
[编辑本段]用法
首先它是动词,而且不同于行为动词,行为动词表示的是可以通过行为来表达的动作（如写,读,跑）,而情态动词只是表达的一种想法（如能,也许,敢）.
用法是：情态动词+行为动词原形
例句：I can read this sentence in English.
我能用英语读这句话.
情态动词是一种本身有一定的词义,表示说话人的情绪,态度或语气的动词,但不能单独作谓语, 只能和其他动词原形构成谓语.
We can be there on time tomorrow.我们明天能按时去那儿.
May I have your name? 我能知道你的名字吗?
Shall we begin now?我们现在就开始吗?
You must obey the school rules.你必须遵守校规.
情态动词数量不多,但用途广泛,主要有下列:
can (could), may (might), must, need, ought to, dare (dared), shall (should), will (would),have (to) ,had better.
can和could的用法
1. 表示能力或客观可能性,还可以表示请求和允许.如：
Can you finish this work tonight?
Man cannot live without air.
— Can I go now? — Yes, you can.
注意：①could也可表示请求,语气委婉,主要用于疑问句,不可用于肯定句,答语应用can（即could不能用于现在时态的简略答语中）.如：
Could I come to see you tomorrow?
Yes, you can. （否定答语可用No, I'm afraid not.）
②can表示能力时,还可用be able to代替.如：
I'll not be able to come this afternoon.
2. 表示惊异、怀疑、不相信的态度.（主要用在否定句、疑问句或惊叹句中）
Can this be true?
How can you be so careless!
This cannot be done by him.
3. “can(could) + have + 过去分词”的疑问或否定形式表示对过去发生的行为怀疑或不肯定.如：
He cannot have been to that town.
Can he have got the book?
4. 用在疑问句及否定句中,表示惊讶,不相信等.
5. cannot```too\enough表示"无论怎样``````也不过分","越``````越好"
二.Should的用法：
1. Should表示劝告、建议、命令,其同义词是ought to；在疑问句中,通常用should代替ought to.如：
You should go to class right away.
Should I open the window?
Should的含义较多,用法较活,现介绍三种其特殊用法.请看下面的句子：
① I should think it would be better to try it again. 我倒是认为最好再试一试.
② You are mistaken, I should say. 依我看,你是搞错了.
③ I should advise you not to do that. 我倒是劝你别这样做.
④ This is something I should have liked to ask you. 这是我本来想问你的.
从以上例句可以看出：情态动词should用于第一人称时可以表示说话人的一种谦逊、客气、委婉的语气.
Should还可以用在if引导的条件从句,表示一件事听起来可能性很小,但也不是完全没有可能.相当于“万一”的意思.从句谓语由should加动词原形构成,主句谓语却不一定用虚拟语气.如：
⑤ Ask her to ring me up if you should see her. 你万一见到她,请让她给我打个电话.
⑥ If you should change your mind, please let us know. 万一你改变主意,请通知我们.
⑦ Should I (If I should) be free tomorrow I will come. 万一我明天有时间,我就来.
此外,Why(or How) + should结构表示说话人对某事不能理解,感到意外、惊异等意思.意为“竟会”.如：
⑧ Why should you be so late today? 你几天怎么来得这么晚?
⑨ — Where is Betty living? — 贝蒂住在哪里?
— How should I know? — 我怎么会知道呢?
⑩ I don't know why you should think that I did it. 我真不知道你凭什么认为这件事是我干的.
2. “should + have + 过去分词”结构一般表示义务,表示应该做到而实际上没有做到,并包含一种埋怨、责备的口气.如： She should have finished it.
I should have helped her, but I never could.
You should have started earlier.
can (could) 表示说话人能,可以,同意,准许,以及客观条件许可,could 为 can 的过去式.
Can you pass me the books?
你能给我递一下书吗 ?
Could you help me, please?
请问,你能帮助我吗?
What can you do?
你能干点什么呢?
Can you be sure?
你有把握吗?
can 和could 只能用于现在式和过去式两种时态,将来时态用 be able to 来表示.
He could help us at all.
他完全可以帮助我们.
With the teacher's help,I shall be able to speak English correctly.
由于老师的帮助,我将能准确地讲英语.
may (might) 可以, 表示说话人同意,许可或请求对方许可.
You may take the book home.
你可以把书带回家去.
May I come in?
我可以进来吗?
May I use your dictionary?
我可以用你的词典吗?
You may put on more clothes.
你可以多穿点衣服.
He said he might lend us some money.
他说他可以借给我们一些钱.
may 否定式为 may not, 缩写形式是 mayn't.
might 是may 的过去式, 有两种用法, 一种表示过去式,一种表示虚拟语气, 使语气更加委婉, 客气或对可能性的怀疑.
He told me he might be here on time.
他说他能按时间来.
Might I borrow some money now.
我可以借点钱吗?
He might be alive.
他可能还活着.
Must 必须,应该,一定,准是, 表示说话人认为有必要做某事, 命令, 要求别人做某事以及对事物的推测.
must 用来指一般现在时和一般将来时, 过去式可用 have to 的过去式代替.
I must finish my work today. 我今天必须完成我的工作.
You mustn't work all the time. 你不能老是工作.
Must I return the book tomorrow? 我必须明天还书吗?
After such a long walk, you must be tired. 走了这么长的路,你一定困了.
He must be the man I am looking for. 他一定是我要找的人.
He had to go because of somebody's calling him that day. 那天他要走是因为有人叫他.
must + have + 过去分词,表示现在对过去事物的推测.
He must have told my parents about it.
他一定把这件事情告诉我父母亲了.
He must have received my letter now.
他现在一定收到我的信了.
It's six o'clock already, we must have been late again.
已经六点钟了,我们一定又迟到了.