在日常的英语学习中，我们遇到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。
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、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.
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。
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。