The course was extremely challenging, but he gritted his teeth, determined to see it through to the end.

gritted 是动词 grit 的过去式，grit 本意是"咬紧牙关"，引申为"下定决心、坚持不懈"。 his 是形容词性物主代词，表示"他的"。 teeth 是 tooth 的复数形式，意思是"牙齿"。 gritted his teeth 是一个固定短语，字面意思是"咬紧牙关"，比喻"下定决心克服困难"。