Discussion Forum—A Way with Words, a fun radio show and podcast about language

Hi!

I'm wondering,

What is the rule for adding an ending on to a proper noun to make it into an adjective?
   (i.e. adding -an for "Scorpian" to say "like a Scorpio
         adding -ian for "Freudian" to say "like Freud"
         adding -ic for "Hellenic" to say "of the Hellenes")

Thanks to anyone who can answer! I'd appreciate it!