at the beginning *在……的开始 **At the beginning of his apprentice days, Alderpaw always felt embarrassed for he was too awkward comparing with his talented sister.
at the mercy of *在……支配下 **At the mercy of his father, Hawkfrost took away one of Firestar’s life. *任由……摆布 **Fern and her fellow rogues’ lives were at the mercy of Slash.