this book is hers=this book is her book（这本书是她的书）
也就是说通常,s代表前面提到过的东西还是不太明白多个例子吧my bike is biggher than yours我的自行车比（你的自行车）大这里不能用than your,但可以用than yours或者than your book yours=your+bike,也就是这个S相当于bike.