on/upon+n/ving，引導時間狀語從句，意思是一……就……..。
相當與when，as soon as，the moment，as 等引導的時間狀語從句。on/upon是介詞，其後跟名詞或動名詞。而when等是連詞，後面跟的是句子。
eg:on his arrival，he phoned her.他一到就給她打電話。=when he arrived，he phoned her.
on和upon引導時間狀語從句，表示一...就得意思，例如
On receiving the letter Tom burst into tears.
