On doing相当于when doing：
一做……
在做……时
如：
On hearing this,she was very excited.她一听到这话,很激动.
doing是动名词,做on的宾语.
on加动词ing形式
On doing相当于when doing：
一做……
在做……时
如：
On hearing this,she was very excited.她一听到这话,很激动.
doing是动名词,做on的宾语.