為什么過去分詞作后置定語前面不加謂語動詞？

過去分詞做后置定語，是定語從句的一種省略。過去分詞直接接在所修飾的名詞后面，省略了定語從句當中的關系代詞和系動詞。例如，The book written by Lu Xun is very popular among teenagers。這個句子可以改寫成,The book which was written by lun is very popular among teenagers.