在日常的英语教学及考试中，同学们在遇到before和until时，往往会将二者搞错。其实，只要我们能把握住两者在使用时本身的含义及主句动词是终止性的还是延续性的，是肯定式还是否定式两大特点，就能很容易地解决这类问题。现详细阐明如下：
两者可互换使用，但含义略有不同。
一。Before表示“在… 之前”，而until表示“直到…才…”，主句肯定表示主句动作的终止时间，主句否定强调动作的起始时间。
1,主句为否定时，谓语动词为终止性动词。常见的动词有open, start, leave, go, arrive, finish, begin, stop, etc. For example:
1), The noise of the street didn’t stop before/until it was midnight.
2), The children won’t come back until/before it is dark.
3), I didn’t leave the lonely boy before/until his mother came home.
2,主句为肯定式，谓语为延续性动词。这类动词有：stand, live, stay, talk, be, wait, etc.
For example:
1). He lived with his parents until/before he graduated from school.
2). I will wait before/until he comes to my help.
3). I shall stay here until/before you come back.
二。下列情况下宜用before不用until.
1。主句为肯定式，谓语为终止性动词时只用before. For example:
1). He fell asleep before I could take off his clothes.
2). We arrived there before it started to rain.
3). The holiday came to an end before I knew it.
4). He almost knocked me down before he saw me.
2。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。For example:
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。
3。如果强调从句谓语动作未发生就发生主句谓语动作，只用before,这时常译为“未及（不等）……就……” For example:
1)We can leave early in the morning before it gets too hot.趁天还不太热，我们上午早点离开。
2)I must write it down before I forget it.趁我还未忘，我必须把它记下来。
4。表示“与其说……倒不如……”,“与其……毋宁……”的意思时只用before.
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。
5．在某些特定句型中用before。
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… For example:
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。
三.下列情况一般使用until
1。主句是持续性谓语动词时，肯定,否定都可以，但意义完全不同，肯定表示动作终止，而否定表示动作开始。For example:
1).We discussed the problem until/before he came back. 我们一直讨论到他回来。
2).We didn’t discuss the problem until he came back. 我们一直等他回来才讨论问题。
2．Not…until…句型尽管在某些情况下可与before互换使用，但在强调句中一般仍用until; 1). It was not until he told me that I knew it.直到他告诉我我才知道这件事。
2).It was not until he finished his homework that he went home.直到完成作业他才回家。
3放于句首时用until. For example:
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。