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1,when表示“當(dāng)……的時候”。從句中既可用延續(xù)性動詞，又可用非延續(xù)性動詞，這些動詞既可以表示動作，又可表示狀態(tài)。從句中的動作既可和主句的動作同時發(fā)生，又可在主句的動作之前或之后發(fā)生。如：
　　when i came into the room, i found him lying there asleep.
　　mary was having dinner when i saw her.
　　when you meet a word you don''t know, consult the dictionary.
　　she was beautiful when she was a girl.

　　如果when從句的主語與主句主語相同，謂語又是be動詞，從句主語和be可以省略：當(dāng)when從句主語與主句主語相同時，用when加分詞可以替代狀語從句。如：
　　when（you are）in trouble, ask her for help.
　　when（i was）sleeping, i never heard a thing.

　　2，as
　　(1)表示"當(dāng)……時"、"和……同時"。常指從句的動作未結(jié)束，主句中的動作就已發(fā)生。從句中多用表示動作的動詞，而不用be動詞或表示感覺、理解、知道這類動詞。
　　as he stood there, he saw two men enter the bar.
　　she dropped the glass as she stood up.
　　as i was walking down the street, an american asked me for directions to the nearest station.

　　(2)用于平行的動作中，表示"一面……一面……"。常指一個主語同時進(jìn)行兩個動作。如：
　　he sang as he worked.
　　the students took notes as they listened.

　　3,while表示"在……的時候"、"在……期間"。它強(qiáng)調(diào)主句的動作與從句的動作同時發(fā)生或主句的動作發(fā)生在從句的動作過程中。while從句中必須是表示動作或狀態(tài)的延續(xù)性動詞。如：
　　never get on or off a bus while it is in motion.
　　the weather was fine while we were in beijing.
　　she called while i was out.