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状语从句在复合句中作状语，修饰主句中的谓语动词、副词、形容词或整个句子。根据句子的含义，它可表时间、地点、原因、目的、结果、条件、行为方式、比较、让步等。状语从句通常由一个连词引起，有时由一个起连词作用的词组引起。 一.时间状语从句 I) when，while和as He entered the room when the meeting was going on. when 通常指一时的动作或状态，可表示从句动作与主句动作同时发生，或先于主句动作;while常用来引导两个同时进行的持续时间较长的动作;as常用来引导两个动作紧接着发生，强调同一时间或一前一后。 When I meet the right chance, I will try my best. While the teacher explained the text, the students listened attentively and took notes He sang songs as he was taking a bath. As spring warms the earth, all flowers begin to bloom. 注：when有时作并列连词意为正在这时、突然，在句末，不能位于句首。从句的谓语动词常用一般过去时，主句通常用过去进行时、过去完成时或be about to结构等。 She was about to leave when Mark arrived so she stayed a little longer. He had been walking under the trees when an apple fell down. | 
