“be 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。另外与be doing sth.when有相同意思的句式还有be about to when.../be on the point of doing sth.when...等。
be doing when
表示“正在做某事，就在这时……”
(1)I was watching the football match when he came in.
当他走进来的时候我正在看足球赛。
(2)He asked me what I would be doing when he came the next day.
他问我当他第二天来的时候我会在干什么。
be about to 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)”
(1)I was about to say when you interrupted me.
我正要说话的时候，你插嘴了。
(2)I was about to go out when the telephone rang.
我正要出去，电话铃响了。
be on the point of doing sth.when
表示“正在做……，就在这时……”
She was on the point of watching TV when there was a power failure.
他刚要看电视，这时突然停电了。
had just done sth.when
表示“刚刚做完某事，就在这时……”
I had just gone to bed when the telephone rang.
我刚刚上床睡觉，电话铃突然响了。