Greek is what is called a declined language. That means that if "rock" is the subject it looks like one thing. If it's the direct object it looks like another. If it's "rock's" (possessive) it looks like another. If it's the subject of a preposition (i.e. - on the rock), it will have another form. And do it all over again for the plural. That's where you get the different spellings. It's a class a noun feminine.
So the subject (and I'm going from memory) is petra (the funny connected "t"s being the "p" sound like that old math friend Pi). The genitive (possessive without getting too technical) is petras (singular). The dative (object of a prep. and some other things) is petra with an iota under the alpha. The accusative (direct object and other things) is petra, again (though it can have other forms, sorry). Pl. subject (nominative) is petrai. then Pl. genitive is Petron (long o, omega, looks like a "w"). Pl. dative is petrais. Pl. accusative is back, confusingly enough, to petras.
I'm sure that cleared it up :)
Uh...err...yessss I...ah....Thanks bass, I think.:)