I'm new to the language and finding the cases (I think that's the word) genitive, accusative and nominative a bit hard to understand. I do know how to change them, but I just do not fully understand it. Any help would be appreciated. :)
Nouns can have different functions in a sentence. One important distinction is that between subjects and objects. Subjects are the nouns which carry out the action, whereas objects are, in a broad sense of the term, recipients of it.
Grammar convention dictates that the subject takes the nominative case, whereas the object the accusative case.
He [subject, nominative] walks. (Sentence with only a subject.)
He [subject, nominative] likes animals [object, accusative]. (Sentence with both a subject and an object.)
In English we don't mark subjects and objects with case inflections, instead the word order indicates which one is which. (Subject comes first, then the object.)
Of course there are exceptions. In Polish, a negated object sentence uses the genitive case instead of the accusative.
I [subject, nominative] know her [object, accusative]. Znam ją [ona, accusative].
I [subject, nominative] do not know her [object, genetive]. Nie znam jej [ona, genitive].
Also, there are certain verbs or verb compounds which necessarily require the object to take the genitive form, although you would expect an accusative.
Some examples are: używać (to use), szukać (to search), słuchać (to listen to), nie ma (there isn't/aren't)...
Furthermore, the genitive case has a lot of different functions that have nothing to do with subjects and objects. For instance, it is commonly used to indicate possession.
To zdanie Bartka [Bartek, genitive]. It is Bartek's opinion. / It is the opinion of Bartek. (the opinion 'belongs' to him)
It is also commonly used with quantifiers.
Tam jest wielu ludzi [ludzie, genitive]. There are a lot of people.
Mam litr wody [woda, genitive]. I have a litre of water.
As you can see, the genitive often corresponds to 'of' in English, but unfortunately not always.
There are also some prepositions that make the genitive case mandatory, like bez (without), z (from), od (from, since), do (to)...
Nie lubię zupy bez soli [sól, genitive]. I don't like soup without salt.
Jadę do domu [dom, genitive]. I'm heading home.
Ona przyjechała z Warszawy [Warszawa, genitive]. She moved here from Warsaw.
Zajmuję się tym od dwóch lat [dwa lata, genitive]. I've been doing this for two years.
I just want to add that you should try not to get too overwhelmed by them! If you say "Widzę pies" instead of "Widzę psa", you'll be understood! Listening to Polish speakers (music, youtube, podcasts) will help your brain get used to what sounds correct, too. If you want to get the tenses correct, you can search any noun or adjective here and get the correct tenses for it : http://odmiana.net/ Powodzenia :)