时间虽是不可再生资源，但是却与我们形影不离，这就造就了 before 很高的出镜率，因为无论是体现更早的时间还是前面的时间，before 都跟时间有密切的联系，而且 before 的多词性使它的用法丰富多彩。
1. 指定时序事件，相当于 earlier than sb /sth，意为”在…以前“，例如：
I must get home before nine o’clock.
Leave your keys at reception before departure.
I like to go for a run before breakfast.
2. 在某些情况下，before 也可以用来指定先后位置，相当于 in front of sb/sth，意为”在…面前（前面）“，对应的是介词 behind，例如：
They knelt before the throne.
Before you is a list of the points we have to discuss.
3. 表示在顺序或安排上某人或某物先于某人或某物，相当于 ahead of sb/sth，例如：
Your name comes before mine on the list.
He puts his work before everything.
4. 表示”当面“，相当于 in the presence of sb，通常 sb 是正在听或等待等的人，例如：
He was brought before the judge.
She said it before witnesses.
注意：在英文写作总，当提到已经写过的东西时，用 above 而不用 before，例如：
As the graph above shows, the rate of inflation has risen by 15%.
如上图所示，通货膨胀率上升了15%。
As stated above, there are four key findings from the study.
作副词时，before 相当于 at an earlier time，in the past 或 already，意为”以前；过去；已经“。
1. 表示现在或说话时间之前的任何时候，此时的用法跟 already 类似，在英式英语中，它们常跟现在完成时连用，例如：
Have you been here before?
I’m so looking forward to the trip. I haven’t been to Latin America before.
2. 表示在说话的某个过去时间之前的任何时候，即表示过去的过去，常跟过去完成时连用，例如：
Suddenly I realized that I had seen him before.
解析：说话的时间是过去（realized），那么 before 是在 realized 之前，表达的是过去的过去，因此用过去完成时（had seen）。
I introduced Tom to Olivia last night. They hadn’t met before.
解析：说话的时间是过去（introduced），那么 before 是在 introduced 之前，表达的是过去的过去，因此用过去完成时（hadn't met）。
3. 与诸如 day，morning，night，week，month 或 year 等时间名词一起使用，表示过去的一个 day，morning，night，week，month 或 year 等，例如：
Two people were ill at work yesterday and three people the day before!
A: Did you graduate in 1989?
B: No, actually, I finished college the year before.
解析：这里的 the year before 译成什么年份取决于说话时的年份，不过保守还是译成”1988 年“比较好，不要译成”前年“，因为如果译成”前年“的话，那么表明说话时所在的年份是1990，但是这个无从考证。
注意：当表示从此刻算起若干时间以前，用 ago，而不用 before，例如：
✔: The letter came a few days ago.
❌: The letter came a few days before.
✔: She was here just a minute ago.
❌: She was here just a minute before.
1. 作从属连词时，before 引导的从句可以放在句首或句尾，放在句首时，通常用逗号把主从句分开，例如：
I always feed the dogs before I have dinner.
Before I have dinner, I always feed the dogs.
2. before 与一般现在时连用谈论将来，例如：
✔: I will call you before I go.
❌: I will call you before I will go.
解析：作为一个规则，时间状语从句中不能出现将来的时态。
3. before 与完成时或过去时连用，强调 before 从句中的动作没发生或可能不会发生，例如：
You can’t watch TV before you have finished your homework.
解析：have finished 这个动作还没发生或可能不会发生。
He lost his temper before I had said a word.
解析：had said 这个动作还没发生或可能不会发生。
Before I had a chance to thank him, he’d gone.
You’re interrupting her before she has even spoken.
解析：has spoken 这个动作还没发生或可能不会发生。
在正式的写作中， before 引导的时间状语从句通常可以省略句子的部分成分，例如：
✔: You can’t go home before finishing the work.
✔: Before finishing the work, you can't go home.
✔: Add the egg before bringing the milk to the boil.
✔: Before bringing the milk to the boil, add the egg.
before 常与 just，immediately，shortly 或 long 等副词以及 days，weeks，months 或 years 等时间名词连用并置于 before 之前，用来强调短暂的时间，例如：
We left just before the final curtain.
He reached Cambridge shortly before three o'clock.
It is significant that he changed his will only days before his death.
有的时候 beforehand 可以代替 before，特别是所涉及的时间没那么具体，常见于非正式的口语中，例如：
Make a list of your questions beforehand.
I wish we'd known about it beforehand.
Before long 意为”不久，很快”，相当于 after a short time，特别出现在写作中，例如：
Before long he took over the editing of the magazine.
We'll be home before long.
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