when表可持续、可瞬间；while之后是持续动作或状态，表同时发生
Strike while the iron is hot。
while sb. was doing, sb. did
While the truck was running, the man fell down on the road.
固定句型：正在做某事，这时突然
while sb. be doing， sb did= sb. be doing ，when sb. did
be about to do sth, when...(正要做某事，这时突然）
I was having a nice dream when the alarm clock rang.
While I was reading a book, the bell rang.
We were about to leave when the telephone rang.
固定句型：一...就...
主语+had hardly done...when 主语 +did
主语+had no sooner done...then 主语+did
I had hardly begun the game when he came in.
I had no sooner begun the game then he came in.
as 表示“当...时候”引导时间状语从句，多强调主从动作同时发生，从句可放主句前或后
As I waited at the stop，I heard a big noise.
She rose up as he entered.
the first time\the moment
