when while as

when while 与 as
     1、when表示“当……的时候”。从句中既可用延续性动词，又可用非延续性动词，这些动词既可以表示动作，又可表示状态。从句中的动作既可和主句的动作同时发生，又可在主句的动作之前或之后发生。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从句的主语与主句主语相同，谓语又是be动词，从句主语和be可以省略：当when从句主语与主句主语相同时，用when加分词可以替代状语从句。如：
     When（you are）in trouble, ask her for help.
     When（I was）sleeping, I never heard a thing.
     2、as
     (1)表示"当……时"、"和……同时"。常指从句的动作未结束，主句中的动作就已发生。从句中多用表示动作的动词，而不用be动词或表示感觉、理解、知道这类动词。
     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)用于平行的动作中，表示"一面……一面……"。常指一个主语同时进行两个动作。如：
     He sang as he worked.
     The students took notes as they listened.
     3、While表示"在……的时候"、"在……期间"。它强调主句的动作与从句的动作同时发生或主句的动作发生在从句的动作过程中。While从句中必须是表示动作或状态的延续性动词。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.