be doing...when...表示“正在做…这时…”。如：
I was sleeping when the telephone rang.我正在睡觉这时电话响了。
类似的句式还有：be about to...when"；正要做…这时…"；。如：
He wad about to go to bed when the telephone rang. 他正要睡觉这时电话响了。
还有 had done...when..."；刚刚做了…这时…"；。如：She had gone to bed when the telephone rang. 她刚睡下这时电话响了。
