The officer was demoted to a regular soldier for breaking the rules.

demoted 是动词 demote 的过去分词形式，表示被动。demote 由前缀 de-（表示"向下、减少"）和词根 mote（来自拉丁语 movere，意为"移动"）组成，字面意思是"使向下移动"，引申为"降职、降低级别"。