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.
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.
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。