Polish is an inflected language and so has cases. Polish nouns and adjectives have different endings depending on the meaning and purpose required in the sentence.
There are seven cases: Nominative, Vocative, Accusative, Genitive, Dative, Instrumental and Locative and these have different uses.
This is a very, very brief outine.
The Nominative or dictionary case is used for the subject of the sentence. It is called the dictionary case because words in a dictionary are written in the nominative.
E.g in the sentence, 'the dog bit the postman, 'dog' is the subject and uses the nominative case.
The Polish accusative case is used: as the object of a sentence.
The object will be changed into the accusative case. (On many occasions the accusative and nominative have the same form so you won't see a difference).
An example of an accusative where the ending changes is piję herbatę 'I drink tea' where' herbata has changed to herbatę.
The Polish genitive case is used:
... to give a negative answer.
So Mam kawę 'I have coffee' becomes Nie mam kawy 'I don't have coffee' in the genitive.
... after expressions of quantity and amount.
So, mleko 'milk' becomes mleka in litr mleka 'a litre of milk'.
... after certain verbs
Examples are szukać 'to look for', zapominać 'to forget' and słuchać 'to listen to'. So słuchać muzyki with muzyka changing to muzyki in the genitive.
... after certain prepositions
e.g. z when it means 'from', dla 'for', od 'from,since' and bez 'without'.'
... to indicate possession.
Possession in English is shown using the apostrophy, for example in the phrases 'my grandfather's' wife' or 'my sister's house'.
Polish uses the genitive case to accomplish the same thing. See possession using the genitive for examples.
After nie maNie ma ... 'there is no ...' is followed by the genitive in Polish.
after numbers greater than four.
The Instrumental case is used:
... after certain prepositions:Pod 'under'. For example Pos stołem 'under the table'.
... with nationality
Jestem anglikiem 'I am English'.
Jestem nauczycielem 'I am a teacher'.