My mother was so tired that she fell asleep when watching TV为什么when后用watching而不用watched
问题描述：
My mother was so tired that she fell asleep when watching TV
为什么when后用watching而不用watched
答
在看电视时睡着了。when后面是过去进行时，省she was.
答
用watching是在when之后省略了 she was，这是允许的
用watched就是省略了she，只是省略主语，这是不被允许的。
答
when watching TV=when she was watching TV 她正在看电视的时候,若用watched就没有“正在”的意思了.