6.狀語從句

　　在句子中起狀語作用的句子叫做狀語從句。狀語從句可以位于句首、句中或句末。位于句首時，從句末通常有逗號與主句隔開；位于句中時，從句的前后都必須有逗號；位于句末時，從句的前面可以不用逗號。狀語從句按意義可分為時 間、地點(diǎn)、原因、條件、讓步、方式、比較、目的和結(jié)果等狀語從句。狀語從句由從屬連詞引導(dǎo)。

　　1.時間狀語從句

　　引導(dǎo)時間狀語從句的連詞有：after，as，before，once，since，till，until，when，whenever，

　　now （that），as soon as，as long as，no sooner…than等。

　　有些名詞短語也可以引導(dǎo)狀語從句，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都可以表示主句中動作發(fā)生的背景，也可以表示主句的動作與從句的動作同時發(fā)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在主句謂語為持續(xù)性動詞的肯定式時，作“直到……為止”解，在主句謂語為瞬間動詞的否定式時作“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，通常可以互換，如：

　　Once you understand this，you will surely make rapid progress in your study.由副詞加從屬連詞no sooner…than，hardly / barely / scarcely…when等引導(dǎo)的時 間狀語從句，如果no sooner，hardly，barely，scarcely前置句首，主語與謂語須用倒裝結(jié)構(gòu)。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.