It dawned on him that he had made a mistake.

dawned 是动词 dawn 的过去式。dawn 本义是"黎明、破晓"，引申为"开始明白、逐渐意识到"的意思。dawn on sb 这个短语表示"某人突然明白或意识到某事"，就像黎明时分阳光逐渐照亮大地一样，想法慢慢变得清晰。