until 意为“到……为止”，before意为“在……之前”。这两个词在用法上有交集，也有不同的方面。
交集方面：
① 如果主句（或简单句）中的谓语动词为持续性动词如stand, live, stay, talk, be, wait,等的肯定式，用until或before均可，但在意义上有一定的差别。如：
She lived in the house until he came back. 她在那所房子里一直住到他回来。（强调“到此为止”）
She lived in the house before he came back. 在他回来之前，她一直住在那所房子里。（强调“在……之前”）
② 如果主句（或简单句）中的谓语动词为非持续性动词（即动作瞬间完成的）如open, start, leave, go, arrive, finish, begin, stop等的否定式，用until或before均可，且意义上没有差别。如：
The moon will not come out until / before the clouds disappear. 云散了月亮才会出现。
不同方面：
① 如果主句（或简单句）中的谓语动词为非持续性动词（即动作瞬间完成的）的肯定式，只能用before，不可用until。如：
He had measured me before I could get in a word.我还没来得及插话,他就把我的尺寸量好了。
She fell asleep before I could take off her clothes. 我还没脱掉她的衣服，她就睡着了。
We arrived there before it started to rain. 我们在下雨前到了那里。
The holiday came to an end before I knew it. 我还没来得及知道假期就结束了。
He can come here before sunset. 在日落之前他能来这。
He almost knocked me down before he saw me. 他看见我之前差点把我撞倒。
另外，主句谓语动词强调动作的迟缓性，只用before,这时常伴有时间段状语或时间段暗示。如：
It was quite a long time before he found the elephant.过了很长时间，他才找到大象。
We had walked a long way before we found some water.走了很长的路我们才找到水。
We had sailed for two days before we saw the land.航行了两天我们才发现陆地。
It was midnight before my brother came home.我哥哥直到半夜才回来。
② 如果主句（或简单句）中的谓语动词为持续性动词的否定式，多用before，一般不用until。如：
He has not done anything before she comes. 在她来之前，他什么也没做。
③ 在It wasn't until ... that 强调结构中不用before。如：
It was not until 1911 that the first of the vitamins was identified.
直到1911年才发现了第一种维生素。
It wasn't until this morning that I heard the news. 直到今天早上我才听到这个消息。
另外：放于句首时用until.如：
Until he finished his homework ，he didn’t go home. 直到完成作业他才回家。
应注意：在使用not until时，若not until放于句首，句子应部分倒装。 For example:
Not until he finished his homework，did he go home. 直到完成作业他才回家。
小结：主句是持续性谓语动词时，肯定,否定都可以用until，但意义完全不同，肯定表示动作终止，而否定表示动作开始。
请看:
1). We discussed the problem until he came back. （强调“到此为止”）
我们一直讨论这个问题到他回来。
We discussed the problem before he came back.
在他回来之前我们讨论了这个问题。（强调“在……之前”）
2). We didn’t discuss the problem until he came back.
我们一直等到他回来才讨论这个问题。（强调“在……之后才……”）
知识拓展：
1. 表示“在…之前”的另外一个英语单词“by”的用法：
by表示在某特定的时间之前或不迟于某特定的时间,意为"最迟在;不迟于"或"在……之前"等。
例子：I will finish my task by nine.我将在九点钟前完成任务。
2. 表示“与其说……倒不如……”,“与其……毋宁……”的意思时只用before.如：
I'd die before I apologized!
我宁愿死也不道歉！
I would give up my job before I agree to be dismissed.我与其被解雇还不如先辞职。
He will die of hunger before he steals.他宁愿饿死也不愿意去偷。
3. 在某些特定句型中用before。
1）It was (not) long before…（不久）过了很长时间才…
It will (not) be long before…（不到）要过很长时间才会…
如：
It will probably not be long before they understand each other.
他们大概过不久就会互相了解。
It was not long before the whole country rose up.
不久全国人民奋起反抗。
It may be many years before the situation improves.
这种状况或许要过很多年才能得到改善。
It was some time before I realized the truth.
过了很长一段时间我才悟出真相。
2） 用于句型“it was 时间段 before …” 表示 “过了多久才…”
It was three days before he came back. .过了三天他才回来。
3） 用于句型 “ it was not long 时间段 before …” 意为“不久就….”
It was not long before he sensed the danger of the position . 不久他就意识到这个位置的危险。
4）用于句型“it will be(not ) 时间段 before….”表示“要过多久/ 不久…才…”
It will be half a year before you graduate from this school .
要过年半年你才会从这所学校毕业。
It will be not long before you find you are wrong .
要不了多久你就会发现你是错误的。
4.until和till只能用于谈论时间。谈论地点或位置时应用as far as 或 up to：
Then you'll be riding with us as far as the village.那么你会坐我们的车一直坐到那个村庄。
I'll keep you company as far as the station.
我将与你结伴到车站。
In the morning I got as far as the sofa and fell on to it.
早晨，我只走到沙发那儿，就一下子倒在上面。
I had thought we might travel as far as Oak Valley, but we onlymanaged the first lap of the journey.
我原以为我们最远能到达橡树谷，结果只走完了第一段路程。
The group has adventured as far as the Austrian alps.
该队伍的探险之旅远至奥地利的阿尔卑斯山。
The garden extends as far as the foot of the hill.
花园一直伸展到山脚下。
We walked up to where his bicycle was.我们一直走到他放自行车的地方。
The water came up to my chest
水淹到了我的胸部。
His sleeves were rolled up to his elbows
他的袖子卷到了肘部。
The sun's rays can penetrate water up to 10 feet.
太阳的光线可以穿透到10英尺深的水下。
Drought and famines have killed up to two million people here.
旱灾和饥荒夺去了这里200万人的生命。(强调数量上直到……)
