when,while,as这三个词都有"当……时候"之意,但用法有所不同,使用时要特别注意.
①when意为"在……时刻或时期",它可兼指"时间点"与"时间段",所引导的从句的动词既可以是终止性动词,也可是持续性动词.如：
When I got home,he was having supper.我到家时,他正在吃饭.
When I was young,I liked dancing.我年轻时喜欢跳舞.
②while只指"时间段",不指"时间点",从句的动词只限于持续性动词.如：
While I slept,a thief broke in.在我睡觉时,盗贼闯了进来.
辨析
①when从句与主句动作先后发生时,不能与while互换.如：
When he has finished his work,he takes a short rest.每当他做完工作后,总要稍稍休息一下.(when = after)
When I got to the cinema,the film had already begun.当我到电影院时,电影已经开始了.(when=before)
②when从句动词为终止性动词时,不能由while替换.如：
When he came yesterday,we were playing basketball.昨天他来时,我们正在打篮球.
③当从句的谓语是表动作的延续性动词时,when,while才有可能互相替代.如：
While / When we were still laughing,the teacher came in.正当我们仍在大声嬉笑时,老师进来了.
④当从句的谓语动词是终止性动词,而且主句的谓语动词也是终止性动词时,when可和as通用,而且用as比用when在时间上更为紧凑,有"正当这时"的含义.如：
He came just as (or when) I reached the door.我刚到门那儿,他就来了.
⑤从句的谓语动词如表示状态时,通常用while.如：
We must strike while the iron is hot.我们应该趁热打铁.
⑥while和when都可以用作并列连词.
Mike is tall while his brother is short.迈克的个子很高,然而他弟弟很矮.
Mary was on her way home when two big boys stopped her.玛丽正走在回家的路上,这时两个大个子男孩拦住她.