Quote Originally Posted by renataf View Post
Hi guys!
I wrote a sentence in Russian "Я поеду в Руссие", and a friend of mine said it was wrong because the correct is "Я поеду в Руссию". She tried to explain me, but I got confused. There´s a preposition (в), so won't I have to use prepositional case?
Note that the "prepositional case" is (sometimes) called by that name NOT because it's the only case that ever uses prepositions, but because it's the only case that can never be used without a preposition. The genitive, dative, accusative, and instrumental cases are sometimes used with prepositions, and sometimes without them.

But incidentally, some English textbooks about Russian grammar refer to the "prepositional case" by a different name: the "locative case," because one of its most common functions is to show where something or someone is located: Павел был на кухне ("Pavel was in the kitchen.")

Mind you, calling it "locative" is potentially confusing because it doesn't always describe physical location -- for example, in the sentence Я читаю статью о планете Марсе ("I am reading an article about the planet Mars."), the nouns планете and Марсе are both in the "locative" case, even though the preposition о/об ("about; in relation to; on the subject of") doesn't refer to location. But on the other hand, calling it the "prepositional" case can also be confusing, since it's not the only case that can take prepositions!