Perhaps the title is somewhat misleading. As far as I can tell Twain never actually wrote a serious article on how to write fiction. Rather, his Rules appear in the context of his famous essay, "Fennimore Cooper's Literary Offences", which is composed of equal parts literary criticism and hatchet job. Twain cites his Rules en passant —
"There are nineteen rules governing literary art in the domain of romantic fiction*—some say twenty-two."and then proceeds to list only 18, some of which appear to be made up on the spot, solely for the purpose of abusing Mr. Cooper's work.
(* It is also worth noting that at the time Twain wrote this essay, all novels were romantic fiction. Even H. G. Wells called his early novels such as The War of the Worlds and The Time Machine "scientific romances." The term romance did not become synonymous with "Harlequin bodice-ripper" until the post-WWII paperback publishing boom.)
Some of Twain's rules are stylistic in nature and simply the rules of good writing, period. E.g.,
These require that the author shall:However, the rules I've seen violated most egregiously and often in the stories I've been reading lately fall into the range of the first eleven, and require some extraction from Twain's wholesale slagging of Cooper. Taking the liberty of editing and elliding Mr. Twain's words, then; the rules of fiction require:
12. Say what he is proposing to say, not merely come near it.
13. Use the right word, not its second cousin.
14. Eschew surplusage.
15. Not omit necessary details.
16. Avoid slovenliness of form.
17. Use good grammar.
18. Employ a simple and straightforward style.
1. That a tale shall accomplish something and arrive somewhere.In music, these are what we call rudiments: the most fundamental and basic skills you need to get down cold before you can even start to think about doing anything more complicated. This isn't an all-inclusive guide, but it's a good start.
2. That the episodes of a tale shall be necessary parts of the tale and shall help to develop it.
3. That the personages in the tale shall be alive, except in the case of corpses, and that the reader shall be able to tell the corpses from the others.
4. That the personages in a tale, both dead and alive, exhibit sufficient cause for being there.
5. (Skipped. Too complex to simplify.)
6. That when the author describes the character of a personage in his tale, the conduct and conversation of that personage shall justify said description.
7. That 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. That crass stupidities shall not be played upon the reader.
9. (Skipped. Not entirely relevant to SF/F.)
10. That the author shall make the reader feel a deep interest in the personages of his tale and in their fate.
11. (Skipped. Debatable.)
Hope this helps. Will write more next week.