be (just) doing sth. ...when...

用于表示当某人或某物正在或正要做某事，就在这时突然又发生了另一件事。

Mary was doing her homework when her mother came in.

当她妈妈走进来的时候玛丽正在做家庭作业。

be (just) about to do sth. ... when ...

表示“正要……这时(突然)……”。when在此为并列连词，意为“就在这时(突然)”

He was about to go out when the telephone rang.

他正要出去，电话铃响了。

We were about to start when it began to rain.

我们刚要出发，天就开始下雨了。