这句话中的 left加后面的句子成分 是过去分词短语 作非限制性定语 修饰 homes 相当于 一个非限制性 定语从句
= which were left without heating during one of the coldest winters in years.
因为与所修饰词 是被动关系 故 用 过去分词
主动关系 是 leaving hundreds of thousands of European homes without heating during one of the coldest winters in years,