He sensed something odd, but he couldn't put his finger on the cause.

put 在这里是动词，意思是"放置"或"表达"。 his 是代词 he 的所有格形式，意思是"他的"。 finger 是名词，意思是"手指"。 on 是介词，表示"在...上"或"关于"。 put his finger on 是一个固定短语，字面意思是"把手指放在...上"，引申为"确切指出"或"准确描述"某个问题或原因。