3．before，since的用法

　　(1)before

　　①意为"在......之前"，置于主句前后均可。

　　Before the sun sets，we must go home.

　　太阳落山前，我们必须回家。

　　②如果before引导的从句位于主句之后，有时不能译成"在......之前"，而要译成"过了多久才......""还未......就"等。

　　The struggle lasted four years before the North won in the end.

　　斗争持续了4年北方才最终获胜。

　　He almost knocked me down before he saw me.

　　他几乎撞到我时才看见我。

　　She left before I could have a word with her.

　　我还没来得及和她说句话，她就离开了。

　　③before常用句型：

　　It will be two years before he leaves the country.

　　再过两年他才会离开这个国家。

　　It was two years before he left the country.

　　过了两年他才离开这个国家。

　　(2)since

　　since意为"自......以来"时，所引导的从句用一般过去时，主句用现在完成时或现在完成时行时。

　　He has worked very hard since he entered the factory.

　　从入厂以来，他工作一直非常努力。

　　since常用于"It's/It has been＋some time ＋since sb.did sth."结构中。

　　It's two years since he joined the army.

　　他参军有两年了。

　　4．表示"一......就......"的引导词

引导时间状语从句，表示"一......就......"之意的引导词有as soon as，immediately