what were you doing when someone knocked at the door?分析一下句型

when引导的是时间状语从句啊,不可以加在其他地方,when不是做修饰的,另外when不是加不加的问题啊,只是引导从句,这个句子当然可以向楼上说的反过来,when后面通常是一般过去式,如果是while的话,通常是过去进行时,例如；what were you doing while someone was knocking at the door?如果掌握好了应该是很好理解的,也不难看懂,加油啦,希望你英语成绩可以在这个假期有很好的提高哦,加油