6.状语从句
在句子中起状语作用的句子叫做状语从句。状语从句可以位于句首、句中或句末。位于句首时，从句末通常有逗号与主句隔开；位于句中时，从句的前后都必须有逗号；位于句末时，从句的前面可以不用逗号。状语从句按意义可分为时 间、地点、原因、条件、让步、方式、比较、目的和结果等状语从句。状语从句由从属连词引导。
1.时间状语从句
引导时间状语从句的连词有：after，as，before，once，since，till，until，when，whenever，
now （that），as soon as，as long as，no sooner…than等。
有些名词短语也可以引导状语从句，如：
every time，the moment，the instant. When I saw her，she was watching closely at the photo on the wall. As the sun rose，the fog dispersed. The instant he received the letter，he started off at once. Every time he went to Beijing，he used to visit the Great Wall. as，when和while都可以表示主句中动作发生的背景，也可以表示主句的动作与从句的动作同时发生，如：
As he stood there，he heard what they were talking about. I stood up and wanted to go out，when she came in. till和until在主句谓语为持续性动词的肯定式时，作“直到……为止”解，在主句谓语为瞬间动词的否定式时作“直到……才”解，如：
Let‘s wait till / until the rain stops. I did not begin work till / until he had gone. once，directly，the moment，the instant和as soon as都作“一…就”解，通常可以互换，如：
Once you understand this，you will surely make rapid progress in your study.由副词加从属连词no sooner…than，hardly / barely / scarcely…when等引导的时 间状语从句，如果no sooner，hardly，barely，scarcely前置句首，主语与谓语须用倒装结构。如：
Hardly had the film started when they came. No sooner had we reached the top of the hill than we all sat down to rest.