时间状语从句,表示“当……过程中”,强调某一时间内主句和从句中谓语动词所表示的动作在同一时期发生.
如：Keep an eye on my sister while I am away.(动作同时发生)
While we were watching TV,MrQing came in.(主句动作开始时,从句动作已在进行中)