when所引导从句既可是延续性的，也可是终止性的。While 所引导的从句，动词只能是延续性的。
He was doing his homework, when the telephone rang.
The telephone rang while he was taking a bath.
I’ll phone you again when I get home.
对比用while. 持续动作，此时从句时态通常相同。
Would you look after the children while I do the shopping?
While I was playing the piano, my sister was doing her homework
不是什么时态都可以。要符合以上的中文。
when的主要用法：
1. 主将从现 I will tell him the news when he comes back.
2. 表突然 I was doing my homework when the phone rang.
3. 其他都是正常用法。
while在中学阶段要记住前后动作同时进行。基本上都是现在或过去进行时。
望采纳！
参考资料：来自百度知道