If you ever find yourself in the land of fairy tales, be wise. Be wary.
All the more if you find yourself in a region where all kinds of fairy tales are true. If you are expecting the Pop Top 20, there will be many surprises.
Here are some warnings. (Part II has been posted1. So has Part III2.)
Combined Parents
If our daughter is under some kind of curse, or someone has predicted misfortune for her, we will not protect her by putting her in a tower.    It won't work.
It won't keep her safe from young men, either.
Also, we will not destroy all the spindles in the kingdom.    It never works, either.
If our daughter gets lost in the woods, and a fox leads her back, we will not begrudge the fox a fat hen as a reward. Especially if there is something -- odd about how we acquired our daughter.
We will invite all the fairies to the christening.
If one of us promises our child -- son or daughter -- to a mysterious being, we will not try to substitute anyone else for the child.    It doesn't work, and it can get the substitute killed.
That also goes if our daughter promised to marry some strange creature.
However, to prevent that, we will not make promises to mysterious beings. Especially promises for cryptic things, because that always turns out to be the child.
Wives And Mothers
I will never, ever, ever express a wish for a child, even one that is -- whatever. Including when it’s a humble statement that I would take whatever child is sent.
If I am a queen and dying, I will not ask my husband to promise to marry only a woman as beautiful as me.    That always ends badly.
Also, I will not ask my husband to marry only a woman who can wear my ring, my gloves, or my gown. That always ends badly.
I also will not ask him to do whatever our daughter asks him to do.    Aspiring stepmothers always figure out how to persuade her to ask him to marry her.
I can ask him to promise to marry only a great princess who won't let being queen go to her head, or to make sure my children never fall under his new wife's power.    He won't keep it, but that's not my fault.
I will behave civilly to all my stepchildren.    And all my children.
I particularly will not send my stepdaughter, or stepson, to a witch in the woods on the grounds we need fire, or a needle and thread. Or tell my stepdaughter to jump into the well after she lost her spindle in it.
I will not be jealous of any daughter or stepdaughter's beauty.
If I do resolve to kill my daughter or stepdaughter -- or daughter-in-law -- I need to do it directly; neither abandonment nor delegation works very well.   
In particular, asking for proof in the form of non-distinctive body parts doesn't cut it.
If I promise my child to a witch, and the witch stops the child to remind her of the promise, I will not send the child off with a riddling answer, so that she doesn't know what happens when the witch seizes her.
I will not steal from a witch while I am pregnant.    That way I can avoid the promise.
I will encourage my children to civil to their half or step sibling.
I will not get into a snit because my stepson doesn't want to marry my daughter.   
I will also not get into a snit because my son wants to marry my stepdaughter.
If my daughter returns from living with a mysterious bear or other animal, and tries to avoid being alone with me, I will let her.    Worried though I am, I don't know how the curse works.
If my son returns from a disappearance and doesn't want me to kiss him, there's magic involved.    I won't do it.    Or let anyone else do it either. Including the dog!
I will not dispose of my married stepdaughter after she has a baby, and replace her with my daughter.
I will not dispose of my stepdaughter -- or niece! -- en route to the wedding and substitute my own daughter.    Especially if the true bride has magical powers.
I will not remarry after being widowed.    That goes double if my son slaughtered a band of robbers, and we're living in their lair, and the sole survivor catches me.
Mothers in Law
If my daughter-in-law is pregnant, I will not use magic to make it impossible for her to give birth.
I will not kidnap my grandchildren and try to frame my daughter-in-law for murdering and eating them.    Even if she can't talk.    Especially if she can't talk.
I will not write to my son and tell him that his wife gave birth to monsters.   
Or kidnap the babies and pretend she gave birth to animals.
If I receive a letter allegedly from my son telling me to have his bride and her children executed, I will hide them away somewhere where I can find them again when he returns and tells me the letter's a forgery.
I will not treat my son's absence as a chance to eat his wife and children for dinner.
Husbands and Fathers
If I am a widower with any sons, I will not remarry.    Ever.   
That goes double if my new wife has a daughter. I have only myself to blame if he refuses to marry the daughter, and her mother turns him into a bear.
If I have only a daughter, however, I'm stuck -- better than her stepmother oppress her than I take it into my head to marry her.   
Though if my daughter asks me to marry a given woman, never ever ever agree.   
Which means I must not promise my wife to do whatever my daughter asks.    Or, for that matter, to marry only a woman who matches her in beauty.
And certainly do not set some condition such as that I will remarry when a pair of iron shoes wears out, or water poured into a boot with a hole in it fills it up, or the like, because it will come true.
I will never ask my daughters how much they love me.    (Though I could always ask her what she means when she says she loves me like salt.)
If offered an exchange where someone does me a favor in return for something expressed as a riddle -- the thing at my house that I do not know I have, etc. -- or for the first thing to greet me upon return, I will turn it down even at cost of my life.    You just know the answer is my child.
If I promised my son (or daughter) to someone, I will not try to replace him (or her) with someone else.
I will refrain from deathbed promises to my wife.
If I do promise my wife on her deathbed to marry only a great princess, who is accustomed to courts, I will take measures to ensure that the bride actually is a princess and not a witch claiming to be a princess who had been driven off from her court.
I will listen to my clever daughter's warning that bringing the king a gold pedestal will only make him ask where the mortar is.
If I hear a prediction that my daughter is going to marry a poor boy, I will not try to kill him, but have him raised suitably. Likewise, for that matter, if it's that my son will marry a poor girl.
If I am a wizard, or ogre, or whatever, and I don't want my daughter to run away with the poor man who showed up, I will give him work that doesn't need her help.    Or, as soon as he obviously had her help in the first task, I will send the daughter off to her aunt for a few weeks.
If my youngest son shows up with something to cure me, and it makes me sick, and his older brothers come up with something that does cure me -- they stole it from him.    I will not order my youngest son's execution on those grounds.
See
Rules For The Land of Fairy Tales, Part II
If you ever find yourself in the land of fairy tales, be wise. Be wary. If you do not fall under Part I’s rules, you may fall under these.
Rules For The Land of Fairy Tales, Part III
If you ever find yourself in the land of fairy tales, be wise. Be wary. If you do not fall under Part I’s rules or Part II’s, you may fall under these.
Never, ever, ever, ever eat anything in fairyland. Or Hades.
This is a great read. But what is the fairy tale about the princess lead home by the fox?