I had never heard of him before he won the prize.

had 是动词 have 的过去式，在这里与过去分词 heard 构成过去完成时态，表示"已经"。 heard 是动词 hear 的过去分词，意思是"听到、听说"。 of 是介词，在这里与 hear 搭配构成短语动词 hear of，表示"听说(某人或某事)"。 had never heard of 是过去完成时的否定形式，表示"过去从未听说过"。