摘要：表示“当--的时候 或“一边--一边 .强调主句的动作和从句的动作同时发生.如: I saw him as he was getting off the bus. 当他下公共汽车的时候我看见了他. The boy was singing a popular song as he was bicycling. 这男孩边骑车边唱歌. As she grew older, she became less active. 当她长大一点的时候.就变得不那么活泼了.
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as if和as though引导的从句所叙述的情况与事实相反时，从句的谓语动词多用虚拟语气。在虚拟语气结构中，用________时态来表示现在的情形；用________时来表示过去的动作或状态；用________时表示正在进行的动作；用________时表示将要发生的动作。
当as if和as though从句的主语和主句主语一致，且从句中含有系动词be的某种形式时该主语和系动词可以省略。
例如：John paused as if(he was)expecting Mary to speak．