Write a Short Story in a Genre You Have Never Tried Before

Genre fiction has structural rules that are not the same as its conventions. The conventions are the clichés; the rules are what make the reader feel the thing the genre is supposed to make them feel. Writing in an unfamiliar genre without understanding the rules produces a story that gestures at the genre rather than actually being one. This explains the rules, generates a premise, writes the opening, and flags the one mistake that immediately signals a first attempt.

I want to write a short story in [genre: horror, crime, literary fiction, science fiction, romance, or thriller] but I have not written in this genre before and do not want to just copy the conventions. Help me: understand the two or three rules that give the genre its structure (not the clichés, the structural elements that make it work), generate a premise that uses those elements without being derivative, and write the opening three paragraphs at whatever register that genre requires. Then tell me the one mistake new writers make in this genre that immediately signals inexperience, and how to avoid it.