while可以翻譯成“當…的時候”，這時候它引導的從句常用用過去進行時，主句可以是一般過去式或過去進行時。那while doing（或when doing）是怎么來的呢？今天我給大家說一下。

當while引導的從句中的主語跟主句的主語是同一個人或物時，這是while引導的從句中的主語和be動詞可省略，故而就變成了while doing。給大家看個句子：

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.

他邊等車邊賣報。

再看下面的句子：

My mother was cooking while I was watching TV.

大家看到這個句子主句的從句的主語不一致，所以不能寫成while doing這個的結構。

注：while doing=when doing，用法是一樣的，例如：

He was selling the newspaper while waiting for the bus.

=He was selling the newspaper when waiting for the bus.

（來源：網絡）