Your story shouldn't rely entirely on flashbacks or explanations of the past. Readers want to live through experience alongside their main characters. On the whole, the most impactful material in your story will be shown in a scene that happens in the story's present. If too much of your story is in flashback, it will feel like it's all explanation, and nothing is actually happening 'now' in the story.
You can use flashback for setting the scene in the beginning.
- This is an explanation used to orient the reader. Don't let it drag on or you'll find yourself in info-dump territory, and you'll ruin the flow of the story.
- The way you write this explanatory set-up can establish the tone of voice of the story - witty, scary, etc.
- It lets you set up hooks or questions in the readers' minds that make them keep on reading.
- Only use this when the reader needs it - when the present prompts it.
- These flashbacks shows us why the hero behaves as he does - the present decision or action wouldn't make any sense if we didn't have the context of the flashback. The flashback is necessary here.
- Overdoing it. You'll end up with a story that's all memories and pondering past events and no real present action - dull.
- Constantly going back to the flashback, or making it too long. You'll labour the point or get repetitive. Especially in a short story, be spare.
- Flashbacks within flashbacks. Very hard to get right, confusing, and it can seem like you're deferring the 'real' story.