Can "on" be used instead of "upon" here? what's the difference between "upon" and "on"?
Native speaker’s answer
Rebecca
Yes! You could use "on discovering" instead of "upon discovering!" The difference is that "upon" is more formal than "on," and "on" is used more frequently in everyday conversations. Ex: On discovering that I had left my hat at home, I went back to get it. Ex: Upon discovering a letter from my uncle, I learned that he had given me his estate.