on/upon+n/ving，引导时间状语从句，意思是一……就……..。
相当与when，as soon as，the moment，as 等引导的时间状语从句。on/upon是介词，其后跟名词或动名词。而when等是连词，后面跟的是句子。
eg:on his arrival，he phoned her.他一到就给她打电话。=when he arrived，he phoned her.
on和upon引导时间状语从句，表示一...就得意思，例如
On receiving the letter Tom burst into tears.
