第四部分 幾個(gè)用法的區(qū)別
1、 can 和be able to的區(qū)別。can在表示“能力”時(shí)與be able to同義。但是can的時(shí)態(tài)形式不如be able to多。在一般將來時(shí)和現(xiàn)在完成時(shí)中只能用be able to。如：
She will be able to dance in a week. 她再過一星期就會(huì)跳舞了。
Nobody has ever been able to do it. 從來沒有人能夠做這事。
在過去時(shí)中，could表示具備某種能力，但不一定做事;而was able to表示努力設(shè)法去做謀事，成功后說明有能力，相當(dāng)managed to do或succeeded in doing。這時(shí)，could和was able to是不能互換的。例如：
When he was a boy, he could swim across the river in fifteen minutes. 他小時(shí)侯，15分鐘就能游過那河。(could表示能力，但不一定這么做)
He was able to swim across the river in fifteen minutes last week.
=He managed to swim across the river in fifteen minutes last week.
=He succeeded in swimming across the river in fifteen minutes last week.
上周他15分鐘就游過了那河。(was able to表示真這樣做了，而且成功了。這里不能用could)
2、must與 have to的比較。 Must 強(qiáng)調(diào)主觀意志，也就是說，必要性來自說話者的主觀意志。 have to 強(qiáng)調(diào)客觀因素。例如：
He must stop smoking. 他必須戒煙。(說話人認(rèn)為他必須)
This is an awful film. ?We really must go. 這部電影糟透了-我們一定要離開這里。(說話人認(rèn)為必須)
You have to go and see the boss. 你一定得去見見老板。(客觀因素)
Catholics have to go to church on Sundays. 天主教徒星期天必須去做禮拜。(客觀因素，如教規(guī)等 )
注意：must not表示“絕對(duì)不可”; don’t have to(或 haven’t got to )表示“不必，沒必要”，兩者意思極為不同。英國(guó)英語中常用needn’t來代替 haven’t got to 或don’t have to,如：
You mustn’t tell Jenny. =Don’t tell Jenny. 你一定不要告訴珍妮。
You don’t have to tell Jenny. =You can if you like but it isn’t necessary. 你不必非要告訴珍妮。
3、need 作為情態(tài)動(dòng)詞與行為動(dòng)詞的區(qū)別。作為情態(tài)動(dòng)詞，need 無過去式和將來時(shí)，故用had to, will have to替代，主要用在否定句、疑問句、條件句中，意思是“有必要”。例如：
He need not come at once. 他不必馬上就來。
If you need go there, please tell me now. 如果你需要去那兒，請(qǐng)告訴我。
Need he come at once? 他必須馬上回來嗎?
(回答要用must, have to, ought to, should等。例如：Need he come at once? Yes, he must/ought to /should/has to. No, he needn’t /doesn’t have to /mustn’t.)needn’t have done表示“本來不必做某事而實(shí)際上已經(jīng)做了”。例如：
We needn’t have waited for him. 我們本來不必等他的。(實(shí)際上已等他了)
用作實(shí)義動(dòng)詞時(shí)，need可用于肯定句、 否定句和疑問句，是“需要”之意。如：
You need to wash your hands. 你需要洗手。
Does he need to take the medicine four times a day? 他需要每日服藥四次嗎?
We didn’t need to see him at all. 我們根本不必要看他(實(shí)際上也沒去看他，請(qǐng)比較needn’t have done的用法)
need做實(shí)義動(dòng)詞時(shí)，后面的賓語如果是動(dòng)名詞，用主動(dòng)形式表示被動(dòng)意義，如果是不定式的被動(dòng)形式，來表示被動(dòng)意義。類似need的這種用法，還有require, want等。例如：
The dinning room needs cleaning every day. =The dinning room needs to be cleaned every day. 食堂需要天天打掃。
My shoes need repairing. = My shoes need to be repaired. 我的鞋需要修理。
4、would與 used to 的區(qū)別 .would 可表示反復(fù)發(fā)生的動(dòng)作或某種傾向。used to表示過去的習(xí)慣動(dòng)作或狀態(tài)，強(qiáng)調(diào)現(xiàn)在已不存在。例如：
She used to work into the night. 過去她常工作到深夜。(現(xiàn)在不了)
She would work into the might. 以前她常工作到深夜。(不表示現(xiàn)在不了)
People used to think that the sun went round the earth. 以前人們總認(rèn)為太陽是圍繞地球轉(zhuǎn)的。(現(xiàn)在不這樣認(rèn)為了)
I used to smoke quite a lot, but I have given it up. 我過去抽煙很兇，現(xiàn)在戒了。
情態(tài)動(dòng)詞的重點(diǎn)和難點(diǎn)是：情態(tài)動(dòng)詞之間的區(qū)別;表示“推測(cè)”的表達(dá)方式;以及“情態(tài)動(dòng)詞+have+過去分詞”的含義。
1、 can 和be able to的區(qū)別。can在表示“能力”時(shí)與be able to同義。但是can的時(shí)態(tài)形式不如be able to多。在一般將來時(shí)和現(xiàn)在完成時(shí)中只能用be able to。如：
She will be able to dance in a week. 她再過一星期就會(huì)跳舞了。
Nobody has ever been able to do it. 從來沒有人能夠做這事。
在過去時(shí)中，could表示具備某種能力，但不一定做事;而was able to表示努力設(shè)法去做謀事，成功后說明有能力，相當(dāng)managed to do或succeeded in doing。這時(shí)，could和was able to是不能互換的。例如：
When he was a boy, he could swim across the river in fifteen minutes. 他小時(shí)侯，15分鐘就能游過那河。(could表示能力，但不一定這么做)
He was able to swim across the river in fifteen minutes last week.
=He managed to swim across the river in fifteen minutes last week.
=He succeeded in swimming across the river in fifteen minutes last week.
上周他15分鐘就游過了那河。(was able to表示真這樣做了，而且成功了。這里不能用could)
2、must與 have to的比較。 Must 強(qiáng)調(diào)主觀意志，也就是說，必要性來自說話者的主觀意志。 have to 強(qiáng)調(diào)客觀因素。例如：
He must stop smoking. 他必須戒煙。(說話人認(rèn)為他必須)
This is an awful film. ?We really must go. 這部電影糟透了-我們一定要離開這里。(說話人認(rèn)為必須)
You have to go and see the boss. 你一定得去見見老板。(客觀因素)
Catholics have to go to church on Sundays. 天主教徒星期天必須去做禮拜。(客觀因素，如教規(guī)等 )
注意：must not表示“絕對(duì)不可”; don’t have to(或 haven’t got to )表示“不必，沒必要”，兩者意思極為不同。英國(guó)英語中常用needn’t來代替 haven’t got to 或don’t have to,如：
You mustn’t tell Jenny. =Don’t tell Jenny. 你一定不要告訴珍妮。
You don’t have to tell Jenny. =You can if you like but it isn’t necessary. 你不必非要告訴珍妮。
3、need 作為情態(tài)動(dòng)詞與行為動(dòng)詞的區(qū)別。作為情態(tài)動(dòng)詞，need 無過去式和將來時(shí)，故用had to, will have to替代，主要用在否定句、疑問句、條件句中，意思是“有必要”。例如：
He need not come at once. 他不必馬上就來。
If you need go there, please tell me now. 如果你需要去那兒，請(qǐng)告訴我。
Need he come at once? 他必須馬上回來嗎?
(回答要用must, have to, ought to, should等。例如：Need he come at once? Yes, he must/ought to /should/has to. No, he needn’t /doesn’t have to /mustn’t.)needn’t have done表示“本來不必做某事而實(shí)際上已經(jīng)做了”。例如：
We needn’t have waited for him. 我們本來不必等他的。(實(shí)際上已等他了)
用作實(shí)義動(dòng)詞時(shí)，need可用于肯定句、 否定句和疑問句，是“需要”之意。如：
You need to wash your hands. 你需要洗手。
Does he need to take the medicine four times a day? 他需要每日服藥四次嗎?
We didn’t need to see him at all. 我們根本不必要看他(實(shí)際上也沒去看他，請(qǐng)比較needn’t have done的用法)
need做實(shí)義動(dòng)詞時(shí)，后面的賓語如果是動(dòng)名詞，用主動(dòng)形式表示被動(dòng)意義，如果是不定式的被動(dòng)形式，來表示被動(dòng)意義。類似need的這種用法，還有require, want等。例如：
The dinning room needs cleaning every day. =The dinning room needs to be cleaned every day. 食堂需要天天打掃。
My shoes need repairing. = My shoes need to be repaired. 我的鞋需要修理。
4、would與 used to 的區(qū)別 .would 可表示反復(fù)發(fā)生的動(dòng)作或某種傾向。used to表示過去的習(xí)慣動(dòng)作或狀態(tài)，強(qiáng)調(diào)現(xiàn)在已不存在。例如：
She used to work into the night. 過去她常工作到深夜。(現(xiàn)在不了)
She would work into the might. 以前她常工作到深夜。(不表示現(xiàn)在不了)
People used to think that the sun went round the earth. 以前人們總認(rèn)為太陽是圍繞地球轉(zhuǎn)的。(現(xiàn)在不這樣認(rèn)為了)
I used to smoke quite a lot, but I have given it up. 我過去抽煙很兇，現(xiàn)在戒了。
情態(tài)動(dòng)詞的重點(diǎn)和難點(diǎn)是：情態(tài)動(dòng)詞之間的區(qū)別;表示“推測(cè)”的表達(dá)方式;以及“情態(tài)動(dòng)詞+have+過去分詞”的含義。