我們知道在比較正式的文體中有時會用 "upon" 代替 "on" 的作用，像是 "be prevailed upon, be expanded upon, depend upon, etc."
I could ever be prevailed upon his doing so.
I could ever be prevailed upon him to do so.
- DaSaGwaLv 77 年前最佳解答
Let me talk about:
The verb used after "prevail upon" to be in infinitive form (不定詞) is NOT because of "upon".
for "I could ever be prevailed upon his doing so", "his doing so" is the object for "upon".
for "I could ever be prevailed upon him to do so", "him" is the object for "upon, "to do so" is the infinitive phrase (不定詞片語) used as object complement (受詞補語) .
Hence, both ways are correct! This tells you that language is very flexible, you cannot use grammar to restrict its use. At the beginning, you might not be able to use it so diversified, but after a while, you will use it at your fingertips.
Now, "on" vs "upon":
Sometimes, both are interchangeable but they are NOT synonyms:
I rely on (or upon) my friends to my furniture.
Even though the above usage (with "upon") is nothing wrong, but it is overkilled. After all, you rely on your friends' strength to move furniture, not SIT or RIDE on them (as "up" might imply). In a word, "upon" is a combination of "up" and "on", and has the meaning of immediately or soon after. Therefore, you use it only when you need both prepositions (up and on) to describe the situation. Similar prepositions like: onto, into, upto, within, without...
However, even though they sometimes is interchangeable, the following examples are NOT:
Hand me the book on the table.
It was the only town on this stretch of the highway.
She will be here on Sunday.
Once upon a time, there was a girl ...
Upon hearing her death, he wept sadly.
She came upon us by surprise.
Hence, using "on" vs "upon" is NOT because "upon" is more formal, but because of its (upon) meaning includes "on".
2013-04-29 10:51:16 補充：
If it is useful to you, then it serves my purpose for being in YK+參考資料： self