"before"表示在某个时间、事情或事件之前发生，"before"常用于描述时间的先后顺序或在某个时间点之前发生的动作。"until"表示某一时间之前一直持续或发生，"until"常用于描述持续的时间段，在某个时间点之前一直持续或发生。
(1)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.
(2)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.
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 control it.
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.
