be doing sth.when...”用於表示當某人或某物正在或正要做某事，就在這時突然又發生了另一件事。

另外與be doing sth.when有相同意思的句式還有be about to when.../be on the point of doing sth.when...等。

1、be doing when

表示“正在做某事，就在這時……”

I was watching the football match when he came in.

當他走進來的時候我正在看足球賽。

2、be about to when

表示“正要……這時(突然)……”。When在此為並列連詞，意為“就在這時(突然)”

I was about to say when you interrupted me.

我正要說話的時候，你插嘴了。

3、be on the point of doing sth.when

表示“正在做……，就在這時……”

She was on the point of watching TV when there was a power failure.

他剛要看電視，這時突然停電了。

4、had just done sth.when

表示“剛剛做完某事，就在這時……”

I had just gone to bed when the telephone rang.

我剛剛上床睡覺，電話鈴突然響了。

when除了用來引導主語從句、賓語從句、時間狀語從句、表語從句和定語從句，還有一種用法值得關注，那就是when引導並列分句，意思是“這時突然;就在那時”，強調另一個動作的突然發生。

常用於以下句型中：

(1)、主語 + be doing… when…意思是“正在做某事這時……”;(2)、主語 + be about to do…when…;(3)、主語+be on the point of (doing) …when…意思是“正要去做某事這時……”

如：

　　I was walking along the river when I heard a drowning boy cry for help. 我正在河邊行走，這時我突然聽到一個落水男孩求救。

　　I was about to leave when it began to rain. 我剛要離開，這時下雨了。

I had just finished my exam paper when the bell rang, announcing the class was over. 我剛做完作業，下課鈴響了。

　I was walking along the streets. Just at the moment I caught sight of a tailor’s shop. 我在街上走。就在那時我看見一家裁縫店