同学你好，该知识点来自沪江网校《英语零基础直达大学四级【双11专享班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学好，
我们以一个具体的例子来看一下：
1. When my mother was cleaning the house, I was reading a book.改成
2. My mother was cleaning the house when I was reading a book.
两句话从语法上来说都正确，没有问题。
不同之处表现在：
句子1的主句是I was reading a book，前面when my mother was cleaning the house是时间状语，强调的是主句“我正在读书”这个动作。
句子2的主句就变成了My mother was cleaning the house，后面when I was reading a book则是时间状语，强调的是主句“妈妈正在打扫房间”这个动作。
祝学习愉快~