Tom found a purse on the ground when he was walking to school.

请问为什么不能用 while?

解答如下：

while 引导的时间状语从句，其谓语动词所表示动作为“持续性/延续性”因此在以上句子可以用while引导时间状语从句。因为 was walking to 为持续性动词。

关于while/when具体用法及区别如下所示：

      when和while引导时间状语从句的区别(1)when表示“当……的时候”。从句中既可用延续性动词，又可用非延续性动词，这些动词既可以表示动作，又可表示状态。从句中的动作既可和主句的动作同时发生，又可在主句的动作之前或之后发生。Mary was having dinner when I saw her.The boy was still sleeping when his mother got home yesterday morning.When the teacher came in, the students stopped talking.(2)while表示“在……的时候”、“在……期间” 、“一边……一边……”。while从句中必须是表示动作或状态的延续性动词。它强调主句的动作与从句的动作同时发生或主句的动作发生在从句的动作过程中。The weather was fine while we were in Beijing.She called while I was out.I was drawing while my brother was reading.He likes reading a newspaper while he is waiting for the bus.(3)如果主句的动作在从句的动作过程中发生，从句常用进行时。While we were swimming, someone stole our clothes.Don't talk so loud while others are working.

特别备注：有时when 和while 可以转换使用。The girl was watching TV when the alien got in.

when和while引导的时间状语从句与这一时态连用。when和while的区别如下。A:when，while都有“当……时候”的意思。when既可表示某一点时间，也可以表示某一段时间 。在when引导的时间状语从句中，其谓语动词可以是延续性的，也可以是非延续性的，可与主句中的谓语动词同时发生，也可在其后发生。I was just reading a book when she came into my room.她走进我房间时，我正在看书。Were you writing when the teacher came in?老师进来的时候，你在写信吗？When he was a child he was always trying out new ideas.他小时候就常常试验一些新的设想。

B:while只能表示某一段时间，不能表示某一点时间。在while引导的时间状语从句中，其谓语动词只能是延续性的，而且也只能与主句中的谓语动词同时发生或存在。While Jim was mending his bike, Lin Tao came to see him.正当吉姆修自行车时，林涛来看他。You can’t do your homework while you’re watching TV.你不能一边看电视一边做家庭作业。

C: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引导的时间状语从句多用一般时态。

While they were talking , the bell rang.正在他们谈话的时候，上课铃响了。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

特别备注：when引导的从句里动作发生的时间相对主句动作发生的时间要短些；while引导的从句里的动作发生的时间相对主句而言更长些。