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.
For example:
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.
我们一直等到他回来才讨论问题.
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:
Antil 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
