It's very wide section of Polish grammar (with emphasis on 'wide'). I think it's impossible to explain it in few sentences. But I'll try to bring you into this topic on an example of nouns. Masculine, feminine and neuter are for singular nouns, the plural nouns divide into 'Masculine personal' and 'Not masculine personal'. Nouns are changed by cases and number classes (singular or plural). In Polish there is 7 cases (mianownik, dopełniacz, celownik, biernik, narzędnik, miejscownik, wołacz which means Nominative, Genitive, Dative, Accusative, Instrumental, Locative, Vocative). http://en.wikipedia.org/wiki/Polish_grammar#Noun_syntax On this site you've got a listing when to use which case. After you'll know which case should be used then you can find the right ending to the word. Table which shows changing of endings of nouns: http://de.wikipedia.org/wiki/Polnische_Grammatik#Allgemeines_Schema If you need some more explanations with examples I recommend you this site: http://polish.slavic.pitt.edu/ There you'll find two books with explanations of Polish grammar. (Start with the version of "Polish grammar in a nutshell"). But in my opinion it isn't a good idea to learn every grammar rule because there are so many of them and even if you achieve all the rules, you'll also need to learn all exceptions (many of them). If I can give an advice, just try to learn Polish grammar from examples and then I hope you'll know which ending fits to which noun (verb/adjective and so on). Wish you good luck :)

It's a huge and complex topic. As Ania wrote, there are seven cases which are used depending on whether the noun is the subject, verb, direct object, indirect object, follows certain verbs, follows certain prepositions and some other things I'm forgetting. There are actually five genders (three masculine) to go along with these seven cases. The links Ania gave are really good. When I started learning, I had a few books, but I remember 'Colloquial Polish' being the most helpful. For some reason things were broken down into digestible pieces. Good luck. There's a steep learning curve at the start, but if you can get past that it will get easier.