8. when 可用作并列连词,表示“这时”;while 也可以用作并列连词,表示“而”、“却”;但 as 则没有类似用法。如: We were about to start when it began to rain. 我们正要出发,这时天开始下雨了。 He likes coffee, while she likes tea. 他喜欢咖啡,而她却喜欢茶...