Here's how you write one.
First, enough of all this hoo-ey (as my father used to say) about the shortstory form. The form is this: I write it; I call it a short story; it is one. End of argument—though it doesn't have to be all that short. I just have to want to call it a “short story.” There's no police involved in this business.
Second, what you need to do is read a lot of all kinds of short stories. This could be viewed as a controversial suggestion by people who fear influence and believe your creativity gets tainted by what you read. For me, though, I first read a short story (“A Worn Path,” by Eudora Welty) long before I ever thought about writing one, and so I never had the chance to write one without knowing that others already existed. You could say that my chance to invent the short story was lost right there, and with it a measure of choice and literary originality. I have therefore had to be consoled that although I couldn't dream up the very first short story, there is perhaps something to be gained—when I sit down to write one—in knowing that there's a precedent for what I'm doing, and that I'm setting off for “short,” not just into the clueless blue, and that I can begin at a very early moment in my cogitations to think about matters of scale, proportion, length, complications of formal features, temporal structure, and the other crucial issues related to, well … the short-ness of things.
Anyway, the impulse to “write short” is probably in the genomes, whether we know there are stories or not. What's inherited is the appetite we develop for pleasure obtainable from all kinds of brevity: pleasure derived from emphasis, from intensity, from abstraction, from self-imposed restraint, from