Several languages make use of so-called cases. In English cases are not used and therefore most people won't know what cases are. But in many other languages, such as German and Russian there are cases. Unfortunately cases make a language a lot more difficult to learn. A case certain 'mode' a word appears in, nouns, pronouns, articles and adjectives usually get a special ending depening on in what case they are.
Cases appear at a certain place in the sentence or after certain prepositions.
In german there are four cases, in Russian there are six cases. But you most likely need to further explanation. Let's use an example to get to know cases. The first case would be Nominative case, the case we will deal with in this chapter, the other will be taught later on. But still, you don't know what a case is so let's go on with it.