Student or Learner
"On hearing it, Sally broke down and wept"
Can I replace the preposition "on" in that sentence with "upon"? What is the difference between them?
You can. In this case, I see no difference at all.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.