There are only three RULES for writing; all else is commentary.
Rudyard Kipling wrote:
What follows is commentary, not rules. These suggestions may help, but what's important is the result -- selling an interesting story.
The archetypical plot consists of a Situation (the protagonist meets a problem), a Complication (the problem makes the protagonist do something about it in a series of actions/reactions of rising intensity), a Climax (the protagonist must solve the problem or be broken by it), a Resolution (the problem unwinds, the protagonist succeeds or fails), and an Anticlimax (left-overs are carted off or explained away). Most (but not all) stories follow this pattern.
One of those nine and sixty ways to construct your story is based on suggestions from the science-fiction writer and teacher, James Gunn:
1. A tale shall accomplish something and arrive somewhere.
2. The episodes of a tale shall be necessary parts of the tale, and shall help to develop it.
3. The personages in a tale shall be alive, except in the case of corpses, and always the reader shall be able to tell the corpses from the others.
4. The personages in a tale, both dead and alive, shall exhibit a sufficient excuse for being there.
5. When the personages of a tale deal in conversation, the talk shall sound like human talk, and be talk such as human beings would be likely to talk in the given circumstances, and have a discoverable meaning, also a discoverable purpose, and a show of relevancy, and remain in the neighborhood of the subject in hand, and be interesting to the reader, and help out the tale, and stop when the people cannot think of anything more to say.
6. When the author describes the character of a personage in his tale, the conduct and conversation of that personage shall justify said description.
7. When a personage talks like an illustrated, gilt-edged, tree-calf, hand-tooled, seven-dollar Friendship's Offering in the beginning of a paragraph, he shall not talk like a Negro minstrel in the end of it.
8. Crass stupidities shall not be played upon the reader by either the author or the people in the tale.
9. The personages of a tale shall confine themselves to possibilities and let miracles alone; or, if they venture a miracle, the author must so plausibly set it forth as to make it look possible and reasonable.
10. The author shall make the reader feel a deep interest in the personages of his tale and in their fate; and shall make the reader love the good people in the tale and hate the bad ones.
11. The characters in a tale shall be so clearly defined that the reader can tell beforehand what each will do in a given emergency.
12. The author shall say what he is proposing to say, not merely come near it.
13. He shall use the right word, not its second cousin.
14. He shall eschew surplusage.
15. He shall not omit necessary details.
16. He shall avoid slovenliness of form.
17. He shall use good grammar.
18. He shall employ a simple, straightforward style.
Elsewhere, Twain wrote: "The difference between the right word and the almost-right word is the difference between the lightning and the lightning bug." Also: "Truth is stranger than fiction, because fiction is obliged to stick to possibilities. Truth isn't."
BUT ALL THIS IS COMMENTARY, NOT RULES.