He regretted not telling her this thing beforehand.
He regretted not telling her this thing in advance.
He regretted not telling her this matter beforehand.
He regretted not telling her this matter in advance.
以上都是可以的.
英语翻译他懊悔没告诉她这件事 （regret doing)不好意思 是 他懊悔没预先告诉她这件事