He expiated his past mistakes by doing community service.

expiated 是动词 expiate 的过去式。expiate 来自拉丁语词根 ex-（表示"完全"）和 piare（"赎罪"），意思是"通过某种行为来弥补或赎罪"。在句中指通过社区服务来弥补过去的过错。