I was reading a book when my mother came home.When my mother came home,I was reading a book .

I was reading a book when my mother came home.
我正在看书,此时我母亲突然回家来了
此为并列句,when “其时”,用以引出一个意想不到的动作
When my mother came home,I was reading a book .
当我母亲回到家的时候,我在看书
此为主从复合句,when 是从属连词“当.时候”