Some things I've noticed about short stories, after years of reading the writers who seem to understand the form better than the rest of us:
A short story has to earn its first sentence more than a novel does. In a novel, the reader gives you a chapter. In a short story, the reader gives you a paragraph. Maybe two. If the first line doesn't create a question worth following, the whole thing stalls before it starts.
Lorrie Moore's Self-Help is written almost entirely in second person. "You" do this, "you" feel that. It shouldn't work. Every writing workshop warns against it. But Moore understood something about the distance second person creates, the way it lets a character observe her own suffering from one step removed.
The best short story endings don't resolve. They shift. Something tilts in the last few lines and the whole story reorganizes itself in your memory. You go back to reread the opening and it means something different now.
Joy Williams once said, "The writer doesn't write for the reader. The writer writes for the page." I've carried that line around for years. It's a reminder that the work comes first, that trying to anticipate what someone else wants to read is a different activity entirely from writing.
Compression is the first thing. A novel can wander. A short story is a room with no hallways. Every sentence either belongs in the room or it's standing in the doorway taking up space.
Annie Proulx's "Brokeback Mountain" covers twenty years in fewer than thirty pages. The time jumps are ruthless. She'll skip four years in a paragraph break. And yet the story feels enormous, like it contains more lived experience than most novels twice its length, because she chose exactly the right moments to stay inside and let the rest fall away.
I'm honestly not sure whether plot matters in short fiction. I've read stories where almost nothing happens and I can't stop thinking about them. I've read stories with clever twists that I forgot by the following week. Something else is doing the work, and I haven't fully figured out what it is.
Dialogue in a short story can't just sound like people talking. It has to sound like people talking while also carrying the weight of characterization and subtext and forward movement, all compressed into a few exchanged lines. That's a lot to ask of a conversation.
One detail, chosen carefully, will do more than a full paragraph of description. Eudora Welty knew this. In her memoir One Writer's Beginnings, she writes about learning to observe, about training herself to notice the single specific thing in a room that tells you everything about who lives there.
A short story that explains its own theme in the final paragraph has failed. The reader shouldn't need the writer to walk back onstage and say what the play was about.