while可以翻译成“当…的时候”，这时候它引导的从句常用用过去进行时，主句可以是一般过去式或过去进行时。那while doing（或when doing）是怎么来的呢？今天我给大家说一下。
当while引导的从句中的主语跟主句的主语是同一个人或物时，这是while引导的从句中的主语和be动词可省略，故而就变成了while doing。给大家看个句子：
He was selling the newspaper while he was waiting for the bus.
=He was selling the newspaper while （he was ）waiting for the bus.（he was可以省略）
=He was selling the newspaper while waiting for the bus.
他边等车边卖报。
再看下面的句子：
My mother was cooking while I was watching TV.
大家看到这个句子主句的从句的主语不一致，所以不能写成while doing这个的结构。
注：while doing=when doing，用法是一样的，例如：
He was selling the newspaper while waiting for the bus.
=He was selling the newspaper when waiting for the bus.
