I am generally a kind, caring person, and like to follow the rules whenever possible.  However, I have Asperger's Syndrome (now called High-Functioning Autism), so I don't always understand things the way most neurotypical people do.  Here are some examples of ways you can explain things to a person with an Autism Spectrum Disoder or another disability (and I might even recommend some of this when explaining things to a neurotypical person.

Explaining the rules:

1.  Don't just give the general rule, but also give a few examples of situations that might be an exception to the rule.  For example, when kids are little, their parents and teachers just tell them, "Don't talk to strangers, don't take anything strangers try to give you, and don't go anywhere with a stranger."  Well, I thought that rule applied in all instances because my parents and teachers just explained the rule in the same way they would explain it to a neurptypical child.  So whenever I was at the park or on the playground, and a kid I didn't recognize asked me if I wanted to play with him or her, I would say no (even if the kid looked like he or she was about my age), because I didn't know the kid.  If my parents and teachers had also said, "But if you're at the park or something, and a kid you don't know asks you if you want to play with him or her, if the kid looks like he or she is about your age, then you can play with the kid even though you don't know him or her.  But if the kid looks older than you, then you should still say no."  Another rule I had a problem with was the rule about how when you hurt someone or something that belongs to someone else, you need to say you're sorry.  If someone hurt my feelings unintentionally, and didn't say, "I'm sorry I hurt your feelings," then I would bully that person because I thought he or she was breaking the rules.  If my parents and teachers had said, "but if you're scared for your safety or someone else's safety, or if someone else is talking when you're talking, then you just need to explain why you did what you did."


2.  Give reasons for why the rule is in place.  For example, about the strangers rule.  Say something like, "Because you don't know them, so you don't know if they're nice or not."  As for the apology rule, say something like, "You need to say you're sorry because it can be a good way to let her know that you didn't mean to hurt her feelings."


Explaining good and bad behavior:

When I was younger, if my brother or I were behaving in a way that was not acceptable (we were usually good kids but even good kids act up every once in a while), my parents would say things like, "It's not ok to act in this way," or, "This kind of behavior is not acceptable," and I don't remember them telling me that some people aren't as quick to learn good behavior as I was, until I started having issues where I was bullying other people for doing things I was taught not to do.  When I was in a private school, I had a kid in my grade who was also on the Autism Spectrum.  I think this kid was on the High-Functioning end of the spectrum, like I am, but I did think he could sometimes be pretty rude.  So I would say mean things to him, because I thought, "He should know better than to act like this!  His parents should've taught him a long time ago that this is not acceptable behavior!"  I think if my parents had sat me down or taken me aside periodically when I was younger and said something like, "Now, Leanne, some kids aren't taught good behavior at home, or aren't as quick to learn as you were, and they might not behave in the way you would expect them to behave," maybe I wouldn't have had as much of an issue. 


I think you get the jist of what I'm trying to tell you.  But my suggestion is, if you're trying to explain something to a person with special needs, don't just explain it the way you would to a neurotypical person (do give them the same rules and boundaries), but be a little more specific in your explanations.