She looked foolish because of the mistakes she [had] made.

foolish 是形容词，由名词 fool（傻瓜）加上后缀 -ish（具有...特性的）构成，意思是"愚蠢的、可笑的"。 had made 是动词 make 的过去完成时形式，had 是助动词 have 的过去式，与过去分词 made 一起构成过去完成时，表示"已经犯下"的动作发生在"看起来愚蠢"之前。