一、在下列情况下，两者可互换用，但含义略有不同。before表示“在……之前”的意思，强调时间先后关系；而until表示“直到……才”的意思，主句是肯定句则表示主句动作的终止时间；主句是否定句则强调主句动作的起始时间。
1.主句为否定式终止性谓语动词。常见的动词是open,start,leave,arrive,finish, stop等。例如：
(1)The noise of the street didn't stop until/before it was midnight.
(2)The children won't come back until/before it is dark.
(3)I didn't leave the lovely boy until/before his mother came home.
2.主句为肯定式、延续性谓语词，这类动词用stand,stay,talk,be,wait等。例如：
(1)He lived with his parents until/before he graduated from school.
(2)I will wait until/before he comes to my help.
(3)I shall stay here until/before you come back.
二、在下列情况下，用before不用until。
1.主句是肯定式、终止性谓语动词，只用before。例：
(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 the 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词的迟缓性，只用before，常伴有时间段状语或时间段暗示。例：
(1)It was quite some time before he found the elephant at all.
(2)We had walked a long way before we found some water.
(3)We had sailed for two days before we saw the land.
(4)The fire lasted about four hours before the firefighters could countrol it.
(5)It was midnight before my brother came home.
(6)It was three months before they met again.
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。常译为“未及”，“不”或“不等……就”。例：
(1)We can leave early in the morning before it gets too hot.
(2)I must write it down before I forget it.
(3)We do want to buy something now before prices go up.
4.表“与其说……倒不如”，“与其……毋宁……”只用before。例：
(1)I would give up my job before I'd agree to be dismissed.
(2)He will die of hunger before he will steal.
5.在某些特定句型中用before。例如：
(1)It was not long before the whole country rose up.
(2)It will probably not be long before they understand each other.
三、下列情况一般区别用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。肯定表动作终止，而否定表动作开始。例如：
(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.我们一直等到他回来后才讨论问题。
2.not…until句型尽管在某些情况下可与before互换用，但在强调句中一般仍用until。
例如：
(1)It was not until he told me that I knew it.
(2)It was not until he finished his homework that he went home.
四、其他技巧
1.before只是陈述以前的事实，用until则着重后来的变化情况：This book was so interesting that he read it for three hours until he realized it.
2.until引出的分句内容是真实的，而before引出的分句的内容可能是真实的，也可以是非真实的：Let's wait until the rain stops.
Pig will fly before he'll become a mathematician.
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