First doubt: When I say or write, I WORK DOWNTOWN, why don't I put any preposition before downtown ? I.e., why must not I use on or on the or in or in the, or at, before downtown, like this: I work ON downtown ???
Because downtown is an adverb.
My second one: When must I use the word or preposition UPON ? In what situations ?
Whenever it is needed, especially with prepositional phrases like depend on, alternatively depend upon.