when 在句首时，主句前要加逗号如 when the doctor camein,we were talking.when 放在句中不用 当when表示“正当……时候”时，when只能在前一分句之后eg. They were watching the World Cup when suddenly the light went out.
while跟过去进行时 when跟过去式或过去进行时
没有区别的
