I've been teaching film students on and off for the past 10 years on how to write and direct short films. Here's a list of what I think one could refer to as rules for the short form:
1. LESS IS MORE
It always is, and I don't mean just in running time. Yes, short films today should be just that - short! From one minute to five at the most. But what I really mean by less is more is do not try to do too much. Do not try to tackle a story which really needs more pages to tell, ie; a feature length idea. It's very important to develop the right idea for a short, and this idea needs to be rather simple. Which leads me to my next rule:
2. SIMPLE ON THE OUTSIDE, COMPLICATED ON THE INSIDE
This ducktails well with my first rule and they are true companions. Simple on the outside allows readers of the script/viewers of the film to understand the story simply and immediately. Complicated on the inside refers to what the film is really about, what the story really deals with. I'll use the feature film JAWS as a perfect example. On the outside the film is about a shark - the shark is killing people and people need to kill the shark. This is simple on the outside. On the inside the film is about much more; greed, corruption, fear, friendship. etc. Being complicated on the inside fulfills us. Gives us something to chew on.
So, when we say a good short film idea needs to be simple - only in production, ie; locations and number of characters. Not simple in scope, as you can see in this Ray film, it is not simple in its scope.
As for a great short film example, here is one by Satyajit Ray:
Have your protagonist want something! Yes, exclamation point as I consider it the most important rule. So does Kurt Vonnegut who says: "every character should want something, even if it's a glass of water."
Conflict. Drama or comedy, you need conflict. When you give your character a goal/something to want, you also must create obstacles - which prevents them from getting what they want, and this bring us to what a story really is: protagonist overcoming obstacles.
So if your character doesn't want anything, and you have no obstacles, you have no story. Period! No ifs, ands or butts. Nada. Niente. Nothing.
5. CREATE REAL CHARACTERS
All great films need to have wonderful, well written characters, but even more so in shorts. Best way to create a real character is by giving them something to want. Also give them flaws. Character flaws. Things to overcome - internally and externally(obstacles).
Everything counts in short films. We have so little time, so little real estate. Get as much as you can out of your location. Think of it as a 3rd character!
Your location should inform us of your character. It can help develop your character. It can also serve as an obstacle, or create tension.
7. SHOW, NOT TELL
Be cinematic. You are not telling a story in words, but with pictures.
Lastly, go back and read that SK quote at the top. Again. He's right. Understand the form, so you don't try and squeeze a feature into a short.