When he came back,what are you doing 为什么用when

When he came back,what are you doing 为什么用when
When he came back,what are you doing 为什么用when

When he came back,what are you doing 为什么用when
当他回来时,你在做什么,主句 what are you doing,“when---”是主句的状语,亦即时间状语从句