①while引导的状语从句中动词必须是持续性.谓语动词多为进行时.或状态动词的一般时.while 的这些用法可用when代替.等于 “at the time that , “during the time that .例如:Please keep quiet while others are studying, ② when除可指一段时间外.还可用来指一点时间.等于 “at the time .也就是说when引出的时间状语从句中的谓语动词可以是终止性的.也可以延续性的.因此主句和从句的谓语可以是一般时.进行时.或完成时.例如:When I went into the lab, the teacher was doing an experiment.He often makes mistakes when he is speaking English. ③as常可与when.while通用.但强调“一边.一边 .例如:As I was walking down the street, I noticed a police car in front of number 37. ④when引导的状语从句中的主语与主句主语一致.主.谓是“主语+系动词 结构时.这时主语和系动词可以省略.例如:When young, he worked for a rich man./ She’ll be here to give you help when necessary. ⑤when有时代替if.引导条件句.意为“如果 .“假如 .例如:I’ll come when (if) I’m free.2.before作连词一般表示时间.意为“在-之前 .但有些句子中这样译就显得别扭.试看以下句子的翻译:He almost knocked me down before he saw me.他几乎把我撞倒才看见我,Before I could get in a word he had measured me.我还没来得及插话.他已经给我量好了尺寸 【查看更多】
