How to Write a Short Story

The term "short" is relative. Some short stories are 3 pages, others are 100. But no matter the length or genre, short stories share a few key elements.

Step One: Problem
Notice that the word 'problem' is singular. That's right; there's only one conflict.
Define your problem. For those who write by the seat of their pants, just keep one problem in mind as you go.

Secondary Problems
It's okay to add a smaller, second problem.
Let's say the main conflict is a teacher who attacks a particular college student's beliefs. If, as a side note, the student is poor, that's fine. But their poverty shouldn't be focused on too much.
You can even use the side conflict to strengthen the main one.
This poor college student might have to move because she can't afford the rent. She gets an apartment farther from campus and must ride the city bus every day to get to school. But, horror of horrors, the teacher she hates rides the same bus every morning. She must endure his rants before she even gets to class.

Step Two: Action or Event
Characters either act to fix their problem, or an event occurs.
In Edgar Allan Poe's The Tell-Tale Heart, the narrator is driven crazy by an old man and his "vulture eye". He takes action by killing the old man.
In Poe's The Cask of Amontillado the narrator is given the opportunity to lead an enemy into a trap.
Does your character take action? If so, how?
Does an event occur? How does it affect the character's life?

Step Three: Wrap it up
Every story, short or not, has a different way of drawing to a close. In most short stories, there is no ending. Questions remain unanswered, life goes on long after the last sentence.

A few questions to get your creative juices flowing;
How is the problem resolved? Is it resolved?
What happens to Main character? To their family? To their ideals?
What does the character learn?
Does it end without really ending? If so, is it a frustrating or a mysterious ending? (Frustrating a reader is always bad, no matter what the reading material.)