1. INTRODUCTION TO A DETECTIVE STORY

2. General Rules for Detective Stories
The criminal must be somebody mentioned in the story. (This is absolutely essential, otherwise the book cannot be called a detective story. A writer should still be cautious because an outright authorial deception must be avoided.)
Supernatural elements are allowable for atmospheric or plot reasons, but they must play no part in the actual solution of the mystery.
Secret passages or hidden rooms are allowable (if the setting allows it), but do not deserve to be used as an explanation of the murder method.

3. General Rules for Detective Stories
Do not use 'foreigners' or other aliens as major characters unless you have some real understanding of their culture and mind-set, and they have some relevance to the plot beyond exotic obfuscation.
Avoid accidental solutions, as they are hardly fair in a story of deduction and the presentation of real clues. And please do not inflict on the poor reader one of those mid-book "Mon dieu, how could I not have seen that before" exclamations which sit like undigested food until the end of the mystery.
The criminal should not be someone you have intentionally presented as totally trustworthy. (If he/she is a liar, at least provide some clue to give the reader a chance to spot that.)

4. General Rules for Detective Stories
All clues must be revealed, although it is perfectly legitimate to disguise them. (But I would draw the line at basing a clue on some misspelling of a word, American vs. British usage, for example, because most books are hardly proofread any more.)
There should be a 'Watson' or some observing point of view that sees but misinterprets the events under investigation. (Only common sense, otherwise where is the drama?)
Do not try to fool the reader with improbable impersonations, such as a woman posing as a man or vice versa and getting away with it by consummate acting ability, especially when they are deceiving people who know them well. (This doesn't even work in Shakespeare.) Especially avoid wigs and false whiskers!

5. Specific Rules/Ideas

6. The detective must be memorable.1.
The detective must be memorable.

7. Fictional detectives are expected to be both clever and a bit out of the ordinary. They must have some small habit, mannerism, eccentricity, interest, talent – anything that sets them apart from the crowd.

8. The crime must be significant.2.
The crime must be significant.

9. Traditionally, the detective novel is constructed around a murder or a great theft. Murder is a crime that cannot be reversed or made amends for; thus, it is a crime worth the detective’s (and the reader’s) time and efforts to solve.

10. The criminal must be a worthy opponent.3.
The criminal must be a worthy opponent.

11. In real life, crimes are committed by ordinary, everyday, sometimes dull and stupid people. However, if fictional detectives are to show off their considerable skills, they must match wits with adversaries of equal cleverness. The mind of the criminal is often the intellectual equal of the detective’s. The conflict becomes a battle of intellects between the detective, his/her opponent, and the reader.

12. 4.
All the suspects, including the criminal, must be presented early in the story.

13. Half the fun of reading a good detective story comes from the mental contest between the reader and the detective in a race to solve the crime. The reader must be able to safely assume that the perpetrator of the crime is one of the main characters in the story, not someone whom the author is going to slip in on the unsuspecting reader in the next-to-the-last chapter.

14. 5.
All clues discovered by the detective must be made available to the reader.

15. Like not springing any surprise suspects, this is another “fair play” rule to which the author must adhere. The reader must be given the same opportunity to solve the crime as the detective, and this means getting the same evidence at the same time it is made available to the detective.

16. 6.
The solution must appear logical and obvious when the detective explains how the crime was solved.

17. The reader must be convinced that he could have come to exactly the same conclusion as the detective. In the end, the reader must see how all the little tidbits of information fit together like so many pieces of a jigsaw puzzle. It is for this reason that detective stories are so appealing.

18. INTRODUCTION TO A DETECTIVE STORY
1. The detective must be memorable.
2. The crime must be significant.
The criminal must be a worthy opponent.
All the suspects, including the criminal, must be presented early in the story.
All clues discovered by the detective must be made available to the reader.
The solution must appear logical and obvious when the detective explains how the crime was solved.