what is the basic principle behind case usage in Russian?

Russian has many different grammar rules which are completely unknown to most English speaking people. In English if we want to refer to an object in a sentence, we may use direct or indirect article (e.g. 'a' or 'the'). Russians like to do the same but instead they attach a letter to the end of a word. So by adding a single letter to the end of a word, you can completely manipulate its meaning. For example, the word ручка (pen) you can change the ending to - ручку, ручки, ручке, ручкой, ручке. All of which have a different function in a sentence.