①when是at or during the time that, 既指时间点,也可指一段时间；
while是during the time that,只指一段时间,因此when引导的时间状语从句中的动词可以是终止性动词,也可以是延续性动词,而while从句中的动词必须是延续性动词.
②when 说明从句的动作和主句的动作可以是同时,也可以是先后发生；while 则强调主句的动作在从句动作的发生的过程中或主从句两个动作同时发生.
③由when引导的时间状语从句,主句用过去进行时,从句应用一般过去时；如果从句和主句的动作同时发生,两句都用过去进行时的时候,多用while引导,如：
a. When the teacher came in, we were talking.
当此句改变主从句的位置时,则为：
While we were talking, the teacher came in.
b. They were singing while we were dancing.
④when和while 还可作并列连词.when表示“在那时”；while表示“而,却”,表对照关系.如：
a. The children were running to move the bag of rice when they heard the sound of a motor bike.
孩子们正要跑过去搬开那袋米,这时他们听到了摩托车的声音.
b. He is strong while his brother is weak.
他长得很结实,而他弟弟却很瘦弱.
as跟when差不多,正当...的时候的意思（AS还有很多意思）

As she left the room she remembered the phone number.
瞬间动词（终止动词）和延续性动词
瞬间动词： 去 走 来 离开 之类的动词,延续动词多用于for引导的完成时态.
I will go to japan next week（短暂）
i have been to japan for three times(延续）
i have been injapan for several years(延续）
i have lived in japan since i was 10(短暂）
i forgot to bring my book with me.(短暂,我把我的书忘在教室了）
i have left my book in the classroom（延续,我把我的书遗忘在教室了.表明状态,现在还留在）