Fiction is about people in trouble. When the trouble is resolved, the story is over.
A short story is a piece of fiction under 15,000 words. It has all the requisite elements of fiction: a protagonist, an antagonist, and a major point of conflict. The bigger the conflict, the stronger the characters. The stronger the characters, the better the story.
Your protagonist is always a reluctant hero. He is flawed. He is dragged out of his comfortable world into uncertainty. He changes internally because he is forced to look at his flaws as a result of the conflict presented by the antagonist. This conflict is the stimulation to his character growth. There should be internal conflict and external conflict in every scene.
A short story conforms to all that is expected of fiction. It is comprised of three acts: Act One: the Setup, Act Two: the Complication, and Act Three: the Resolution.
Act One shows the protagonist before the trouble starts, in his comfortable world, but with myriad problems. Act One ends when the protagonist is so tired of avoiding the impending problem that he believes it is easier to fix the problem than to continue to avoid it. This is when he embarks upon his quest. By the end of Act One, all the major players have been introduced, as well as the major point of conflict.
Act Two complicates every tiny point of conflict introduced in Act One. At the end of Act Two, the protagonist and reader alike are certain he will never be able to fix the problem. This is the darkest moment.
In Act Three, the conflicts begin to resolve as a result of the protagonist getting smarter. In the climax, he deals, once and for all, with the external conflict, and he takes a good look at his internal flaws. This is when he either succumbs to his failings or overcomes them. The reader is cheering for him to overcome his flaws, but characters do whatever they do. The point is that he must look at himself and be changed by what he sees.
In the final analysis, readers will remember what happens to the protagonist internally, which is ultimately more important than what happens to the external problem.
A short story can be told from any point of view, can include any number of characters, can span any length of time. There is no room for subplots, so stick to one good guy, one bad guy, and one main point of conflict. Give your characters passion, memorable names, quirks, angers, frustrations and depth. Include lots of sensory imagery, so the reader can be in the scene with the character, and reveal your character’s nature through the use of facial expressions and gestures. Differentiate the characters from each other, and from you. Give them a serious problem, throw them off the deep end, and watch them work their way out of it, given who they are and what they do.