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.