What are the main rules that they must follow when using their powers?
The Halliwell sisters are not allowed to use their magic for personal gain unless its in self defense. If they do try to use their magic for this reason, as Prue and Phoebe did when trying to save Piper from the Arroyo Fever, there will always be consequences. Phoebe has become quite adept at writing spells that get around this clause though. The other important rule they must follow is that their magic can only be used to protect the innocent and not to punish the guilty. Phoebe learned this the hard way when she discovered that her future self was going to be executed for killing a man with her power.
