类别

作    用

例     句

as

as表示“当……的时候”，往往和when/ while通用，但它着重强调主句与从句的动作或事情同时或几乎同时发生。

She came up as I was cooking.(同时)

The runners started as the gun went off.(几乎同时)

when

(at or during the time that )既可以表示在某一点的时候，又可表示在某一段时间内，主句与从句的动作或事情可以同时发生也可以先后发生。

It was raining when we arrived.(指时间点)

When we were at school, we went to the library every day.(在一段时间内)

while

while意思是“当……的时候”或“在某一段时间里”。主句中的动作或事情在从句中的动作或事情的进展过程中发生，从句中的动词一般要用延续性动词。在when表示a period of time时，两者可以互换。

Please don’t talk so loud while others are working.

He fell asleep while/when reading. Strike while the iron is hot.(用as或when不可，这里的while意思是“趁……”)