现在分词作后置定语,修饰message, 相当于定语从句which is reading,与主句谓语动词同时发生
1.When I got back home I saw a message pinned to the door (reading) "Sorry to miss you ;will call later."
回答：不是 "which is reading" 而是...
关于非谓语动词的两句话现在分词作后置定语,修饰message,相当于定语从句which is reading,与主句谓语