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when while 與 as 1、when表示“當……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……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. |
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