I was so nervous that my hands were shaking before I set foot in the exam hall, but I felt a surge of confidence the moment I set foot on the stage for my presentation.

set 在这里是动词，意思是"放置、安置"。 foot 是名词，指"脚"。 in 是介词，表示"在...里面"。 set foot in 这个短语字面意思是"把脚放进...里面"，引申为"进入(某个场所)"。 on 是介词，表示"在...上面"。 set foot on 这个短语字面意思是"把脚放在...上面"，引申为"踏上(某个表面)"。