While 只能跟延续性动词，不可跟短暂性动词。 ①在…..期间 (从句用过去进行，主句用一般过去式) Eg:While we were talking, he came in. 爷爷在看电视期间，睡着了。 Grandpa fell asleep while he was watching TV. ②对比，然而 China is a developing country while America is a developed one. ③两个动作同时进行，主从都用进行时。 My father was watching TV while my mother was cooking.