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Sometimes the "easiest" assignments are actually the hardest until you break them down into their components. Finding and translating a Japanese short story into English is a good example of one of those assignments that can leave you completely baffled at how to get started.
So, let's break down the task into components and figure out how to get the materials you need to complete your assignment.
You will need to choose a Japanese text that is "complete" and approximately 15-20 pages long.
This generally means that you are looking for a short story 短編小説 (tanpen shosetsu)
A published English language translation should not exist.
There is no definitive list of "untranslated Japanese short stories" so you need to work by elimination.