一、 before用作连词
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