【When与while 的区别与联系】
一、when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.
二、while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.
三、【辨析】
1、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)
2.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.
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.【另外,when和while的区别还在于：while引导的时间状语从句多用进行时 态,而when引导时间状语从句,主句用过去进行时,从句应用一般过去时.
例如：1>、While they were talking ,the bell rang.
正在他们谈话的时候,上课铃响了.
2>.When the teacher come in,we were talking.【当此句改变主从句的位置时,应该用While we were talking ,the teacher come in.我们正谈话的时候,老师进来了.（这是一个复合句.其中while we were talking 是由while引导的时间状语从句.这里while表示主句的动作是发生在从句所表示的时间之中.显然,while 所表示的是一段时间,不是表示一点时间.）
【试比较：
（1）、They were sweeping the floor when the teacher came in.
老师进来时（这一时刻）,他们在擦地板.
（2）、While they were sweeping the floor,the teacher came in.
他们正擦地板时（这段时间）,老师进来了.】
(3) I was doing my homework when my mother came back home yesterday evening.昨天晚上妈妈回家的时候,我正在做家庭作业.】
【注意】：如果主句和从句的动作同时发生,两句都用过去进行时的时候,多 用while引导.例如：They were singing while we were dancing.】
5.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.
6.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.
7.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.
【【When与while 】】
1.while后面跟 时间段,when后面可跟 时间点 或 时间段(一般跟时间点).
2.while中主句和从句的 动作 必 同时发生,when中主句和从句 动作 可以是 同时发生,也可以是 不同时发生.
3.while中谓语动词必用延续性动词,when中可用延续性或瞬间性.
4.while可表示转折,when不行.
