Upon是一个介词，意思是“在…之上”，“在…之后”。在英文中，它通常用于表示时间关系，表示某一件事情在另一件事情之后发生。例如：
Upon my arrival at the airport, I reapzed that I had left my passport at home.
我到达机场后才发现我把护照留在家里了。
Upon receiving the news, he became very upset.
他一听到这个消息就很激动。
Upon further investigation, it was revealed that the witness had ped.
经过进一步调查，发现该证人撒了谎。
Upon my arrival at the airport, I reapzed that I had left my passport at home.
我到达机场后才发现我把护照留在家里了。
Upon receiving the news, he became very upset.
他一听到这个消息就很激动。
Upon further investigation, it was revealed that the witness had ped.
经过进一步调查，发现该证人撒了谎。