While we were talking, the teacher came in.When you call,your sister doing?
when, while这两个词都有"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.
a. when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)
b. when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.
c. 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.
d. 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.
e. 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.
f. while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.