Dzień dobry! I have been studying Polish language and am quite confused about the many cases.
I have learned nominative, narzędnik, biernik and dopełniacz. However, I don't know when I am supposed to use each.
If I am giving any sentence in English and need to translate it into Polish, I will have no clue which form to use!
Is there an easy explanation to help me?
No there is no easy distinction. learning how to use them correctly is difficult even for advanced students and takes a lot of study and practice. These cases are all explained in detail in grammars of the Polish language, like Oscar Swan's. He has put a short version of his grammar on his website here: polish.slavic.pitt.edu/firstyear/nutshell.pdf
Here is a brief rundown of the usage for each one (this is my website, which I haven't touched for ages, if native speakers would let me know if there are any errors please)
polish-dictionary.com/polish-accusative-case - Accusative Case - Biernik
polish-dictionary.com/polish-genitive-case - Genitive Case - Dopełniacz
polish-dictionary.com/polish-instrumental-case - Instrumental Case - Narzędnik