upon arriving home ,the girl discovered that she had left her bag at her friend's.这里面的upon是什么意思?

问题描述:

upon arriving home ,the girl discovered that she had left her bag at her friend's.这里面的upon是什么意思?

upon=on 介词
表示当...时候,相当于时间状语从句
当到家的时候,这个女孩子发现她把包落在她朋友家里了