when 与while 讲解
We were having an English lesson when the headmaster came.
句中when表示at that time(校长到来的那个时候), 引导的从句动作是突然发生的,动词用一般过去式，而主句要用进行时态。再如：
（1）I was doing my homework when the telephone rang.
（2）They were fishing when someone called for help.
When we were having an English lesson, the headmaster came.
= While we were having an English lesson, the headmaster came.
引导的句子是进行时态或持续性状态时，可以用when 或while, 表示during the time.
(1) My mom is reading while my dad is watching TV.
(2) Please wait here while I go back to get the umbrella.
(3) I like red while he like blue.
句子（1）和（2）前后是在同一时间，不同人做不同事情，形成对比，句子（3）表示前后的对比，用While,表示“然而”。
练习：选用when/while 填空
They were having fun on the playground ______ it rained.
Lucy likes singing _____ Lily likes dancing.
The door bell rang _____ I was taking a bath.
_____ the door bell rang, I was taking a bath.
Mr Lee was thinking _____ the students were talking at this time yesterday.
答案：
when
while
when 或 while
When
while