浅议before的用法
在高中英语学习中，before是一个较为常用的虚词，它既可用作介词又可用作副词，还可以用作从属连词。其作为从属连词的用法是整个高中阶段语法学习的重点和难点。近些年各地的高考都倾向于考查这一用法。现将其作为从属连词的用法归纳如下：
1. 表示“在……之前”，用来说明主从句所描述动作发生的顺序。
Mary had cleaned the classroom before she went home.
Mary在回家之前已经将教室打扫干净了。（before在这里表示Mary是先打扫教室，然后才回家。打扫教室的动作发生在回家之前。）
They had got everything ready before I arrived.
在我到达之前他们已经把一切都准备好了。
在做决定前，你一定要学会考虑自己的感觉与理智。
解析 before you reach。这里是指要先考虑到自己的感觉和理智，然后再做决定。
2. 表示“过了……之后……才……”，强调从句的谓语动词所表达的时间、距离很长或花费较大。
She and her family hid away for nearly twenty-five months before they were discovered.
她和她的家人躲藏了近25个月才被发现。
They worked day and night about three days before everything returned to normal.
他们不分昼夜地工作了三天直到一切恢复正常。
3.表示“还没来得及……就……”，目的在于强调从句所表示的动作发生之前，主句动作就已经发生，着重体现主句谓语动词所表达的时间、距离短或花费的精力较小，常与情态动词can/could连用。如：
She was angry before I could explain to her.
我还没来得及向她解释，她就生气了。
例2 ―Why didn’t you tell him about the meeting？ （2006年高考四川卷）
―He rushed out of the room I could say a word.
A. before B. until
C. when D. after
解析 A。这里表示我还没来得及说句话，他就冲出了房间，故正确答案应为before。
例3 Someone called me up in the middle of the night， but they hung up I could answer the phone. （2000年高考全国卷）
A. as B. since
C. until D. before
解析 D。本句意为“有人在半夜打电话给我，我还没来得及接电话，他就挂断了”。before在这里就表示“还没来得及……就……”。
例4 The girl had hardly rung the bell ， the door was opened suddenly， and her friend rushed out to greet her. （2010 高考福建卷）
A. before B. until
C. as D. since
解析 A。根据rung the bell和the door was opened发生的先后，可以确定答案为A。句子的含义是：女孩还没来得及按门铃，门就突然打开了。她的朋友们一拥而上去欢迎她。
例5 It was several minutes ， what he was saying. （take） （2014年高考湖北卷）
过了好几分钟，我才理解他说的是什么。
解析 before I could take in/before I took in。before引导状语从句表示“……才……”。
4.表示“趁着……，以免”。
He made a mistake， but then he corrected the situation before it got worse.
他犯了个错，但他在酿成事故之前就纠正了。
You’d better write the numbers down before you forget them.
趁你还没有忘记，赶紧把这些号码写下来。 5.表示“一……就”“刚刚……就……”，主句部分常与hardly或scarcely连用，且主句部分常与过去完成时连用，这里的before等同于when。当hardly， scarcely等含有否定意义的词位于句首，主句需部分倒装。
He had hardly gone to bed before/when the telephone rang.
=Hard had he gone to bed before/when the telephonel rang.
他刚刚睡下，电话就响了。
She had scarcely opened the door before/when her daughter ran towards her。
=Scarcely had she opened the door before/when her daughter ran towards her。
她刚一开门，她女儿就跑了过来。
6.表示“不知不觉多少时间已经过去了”。
He was so absorbed in his painting that four hours had passed before he knew it.
他如此专注于绘画，不知不觉四个小时就过去了。
Time went fast and four months went by before he knew it.
时间飞速地流逝，不知不觉四个月过去了。
例6 Several weeks had gone by ， I realized the painting was missing.
A. as B. before
C. since D. when
解析 A。此处before表示“不知不觉……时间已经过去了”。此句意为“当我发现画不见了，时间已经过去了好几个星期”。
7.表示“宁可，宁愿”，相当于rather than，且在主句部分常与would连用。
Liu Hulan would die before she gave in to enemies.
刘胡兰宁愿死也不愿向敌人屈服。
8. 用于句型“it was （not）+时间段+before...”表示 “过了……才……”“没过……就……”，从句部分常与一般过去式连用。
It was some time before we realized the truth.
过了一段时间我们才明白真相。
9. 用于句型“it will be+时间段+before...”表示“过多久/不久……才/就……”，从句部分常与一般现在时连用。
It will be two months before we graduate from this school.
再过两个月我们就要从这所学校毕业了。
The field research will take Joan and Paul about five months. It will be a long time before we meet again.
该领域的研究将占去琼和保罗大约5个月的时间，我们要过很长时间才能再见到他们。
例7 ―How long do you think it will be ，China sends a manned spaceship to the moon？
―Perhaps two or three years.
A. when B. until
C. that D. before
解析 D。考查“it will be+时间段+before...”句型。此句意为“你认为要过多久中国才会向月球发射载人宇宙飞船？”