During the race, he stumbled and fell flat on his face in front of everyone.

fell 是动词 fall 的过去式，意思是"跌倒、摔倒"。 flat 是形容词，意思是"平的、平坦的"，这里形容摔倒时的状态。 on 是介词，表示"在...上面"。 his 是形容词性物主代词，指代"他的"。 face 是名词，意思是"脸、面部"。 fell flat on his face 是一个固定短语，字面意思是"脸朝下平摔"，引申为"彻底失败"或"当众出丑"的意思。