He is putting up some signs asking for old booksHe is putting up some signs asking for old books and magazine.为什么要用ask的ing形式

He is putting up some signs asking for old booksHe is putting up some signs asking for old books and magazine.为什么要用ask的ing形式
He is putting up some signs asking for old books
He is putting up some signs asking for old books and magazine.为什么要用ask的ing形式

He is putting up some signs asking for old booksHe is putting up some signs asking for old books and magazine.为什么要用ask的ing形式
因为前面有is,所以后面所有的动词都用ing形式