before详细用法

after用法小议
通常用于过去时、现在时（主将从现）或完成时态的句子中。

1. 1. 用作介词，与一段时间连用表示“在…之后”时，注意它与 in的区别：after表示以过去时间为起点的一段时间以后，且通常与过去时态连用；而in则表示以现在为起点的一段时间以后：He came back after two days. 他两天后就回来了。

He will come back in two days. 他过两天就回来。

但 after 若是与一点时间连用，则可与过去时态也可与将来时态连用：He came back after 4 o’clock. 他是四点钟以后回来的。

He will come back after next month. 他将在下个月以后回来。

eg. 2. I played computer games after I had finished my homework. 做完功课后，我玩电脑游戏。

(从句的动作发生在主句的动作前，因此，主句用一般过去式，从句用过去完成时．) Before和after引导的时间状语从句中，由于before和after
本身的词义已经能够表明动作发生的先后，所以主句可以用一般时代替完成时，也可以用完成时。

I knew/had known her long before you knew her.我认识她早在你认识他之前。

After you finish/have finished all your homework,you can watch TV this evening.你完成所有的作业后，今晚可以看电视。

2. 用作连词引导时间状语从句时注意：若主句用将来时态 (或含有将来意味)，则时间状语从句要用一般现在时态 (表将来意义)表将来意义，而不能直接使用将来时态：他回来后我就走。

正：I’ll leave after he comes back.
误：I’ll leave after he will come back.
3. 用作副词，表示“后来”、“ (一段时间)以后”，通常放在表示时间的副词之后：soon after 不久以后 two days after两天后
The plane was hijacked soon after it took off.
Two days later he was gone.
4. 用于习语after all 终究，毕竟：I was right after all. 毕竟我是对的。

After all, he’s only six years old. 毕竟，他才六岁。

before用法小结
请先看2005年广东省的一道高考题：
The American Civil War lasted four years _______ the North won in the end.
A. after
B. before
C. when
D. then
[解析] 答案为B。

本题考查连词before的用法。

before含有“在……之后才……”之意，根据本句含义“美国南北战争持续了四年，北方才最终取得胜利”，可知本题应选B。

before一词含义广泛，是热点词汇之一，但部分考生对该词的用法模糊不清。

为了帮助大家掌握，现就对它的用法归纳如下：
一、 before用作连词(conjunctions)
“在……之前”，常与一般现在时、过去时连用。

1． before用作连词时，意思是“在……之前；还没来得及……就”。

例如：
Before she could move, she heard a great noise, which grew to a terrible roar. 她还没来得及动弹，就听见有很响的声音，接着就成了可怕的隆隆轰鸣声。

Before I could say a word, she had stormed out of the room. 不等我说一句话，她就冲出了房间。

Don’t count your chickens before they are hatched. 不要高兴得过早。

2．用于It + be + 时间段 + before...句型中，意思是“在……之后才……”。

例如：
It will be five years before I come back.
我五年后才能回来。

It was not long before the enemy was driven out of their
country. 不久之后，敌人就被赶走了。

如果主句中用否定式，则表示“不多久……就”。

It won’t be long before they understand each other. 他们不久就会互相了解的。

3．还有“趁……（还没有）”之意。

例如：
They are thinking of buying the house before the prices go up. 他们正在考虑趁房价未涨时把房子买下来。

Do it before you forget it. 趁早动手，以免忘了。

二、before用作介词
before用作介词时, 可以指时间“earlier than在……之前”，也可以指位置“in front of在……之前”。

例如：
There is nothing that has not been bitter before being ripe. 凡事在成熟前，都是有苦味的。

Don’t put the cart before the horse. 不要本末倒置。

He was brought before the judge. 他被带到法官面前。

三、before用作副词
before用作副词时，意思是“at an earlier time / in the past以前”。

例如：
I’ve seen the film Titanic before. 我以前看过《泰坦尼克》这部电影。

I have never seen such beautiful flowers before. 我以前从未见过如此美丽的鲜花。

四、短语before long与long before的区别
before long是“很快，不久”的意思, 可以用于各种时态。

例如：
Before long, he went to America. 不久，他就去了美国。

I hope to see you before long. 我希望不久后再见到你。

而 long before是“很久以前”的意思，一般用在过去时或过去完成时的句子里。

例如：
She said she had read the novel long before. 她说她很久以前就读过这部小说。

That happened long before. 那件事发生在很久以前。

[巩固练习]
1. It _______ long before we _______ the result of the experiment.
A. will not be; will know
B. is; will know
C. will not be; know
D. is; know
2. It was about five hours _______ the workers shipped all the goods.
A. since
B. when
C. after
D. before
3. It was evening _______ we reached the little town of Winchester.
A. that
B. until
C. since
D. before
4. Scientists say it may be five or six years _______ it is possible to test this medicine on human patients.
A. since
B. after
C. before
D. when
5. It was some time ____ we realized the truth.
A. when
B. until
C. since
D. before Keys:1-5: CDDCD。