If one thing is upon another, it is on it.
on
He set the tray upon the table.
他把盘子放在桌上。
He bent forward and laid a kiss softly upon her forehead.
他俯下身,温柔地吻了一下她的前额。
You use upon when mentioning an event that is followed immediately by another event.
The door on the left, upon entering the church, leads to the Crypt of St Issac.
一进入教堂,左侧的门就通往圣以撒之墓。
You use upon between two occurrences of the same noun in order to say that there are large numbers of the thing mentioned.
Row upon row of women surged forwards.
一排又一排的妇女向前涌去。
If an event is upon you, it is just about to happen.
The long-threatened storm was upon us.
那场酝酿已久的暴风雨即将来临。
The wedding season is upon us.
婚礼旺季即将来临。