What are all the different cases in Russian used for? How do you use them ?

There are 6 different cases in Russian, lets go through the first one: 1) nominative (именительный падеж)- this is when the noun of a sentence is the subject of the sentence. So it answers the questions 'what' and 'who'. e.g. so if we have the sentence - Собака грызёт кость. Собака will be the subject and in the nominative case. (what?/ who?)- the dog. so it does answer our question.