张祥麟编著《英语学习难点200则》（中国标准出版社1990年4月第1版）P309：

A. 在从句中的长动作（包括延续性动词的一般时和进行时）的过程中，有主句的短动作（瞬间动词的一般时）发生时，when和as都可以。例如：

As/When I was walking to school, I saw Mr. Johnson.

Mother called to me as/when I read a novel.

B. 两个一般时的长动作相并进行的时候，as和when都可以。例如：

He sang as /when he worked.

C. 两个进行时的长动作相并进行的时候，when可以，as不可以。例如：

Mary was cooking supper when her brother was playing chess.

D. 两个短动作或事件同时紧接发生时，as和when 都可以。例如：

He arrived just as /when I stepped out of the room.

E. 从句以系表结构表示状态时，when可以，as 不可以。例如：

When it is wet, the buses are crowded.

I left school when it was dark.

F. 不论主句还是从句，只要有一个用了完成时态 ，只可以用when。例如：

Hardly had I finished my composition when the bell began to ring.

I’ll go sight-seeing with you when I have got well.

G. 如果从句用瞬间性动词，主句用延续性动词，从句只能用when引导。例如：

When he came this morning, we were having an English class.

Father was writing something when I left home.