Bambino, citam ja, i ako naletim na neku gresku sigurno cu ti javiti. Sad cu pokusati da ti malo pomognem.
As you could see in Croatian, one of the most difficult things to understand are cases of nouns. Let's put aside the unregular ones, and try to explain with normal.
For example, I will use word PROFESOR = professor, teacher (male gender, singular).
1. NOMINATIV -
Profesor
Example:
Ovo je moj profesor (This is my professor).
2. GENITIV -
Profesora
This is used in sentences where you want to show some belonging.
Traži pomoć od profesora (Ask professor for help).
3. DATIV -
Profesoru
This is used when you give something and directly interact.
Treba da vratim ovu knjigu mom profesoru (I need to return this book to my professor).
4. AKUZATIV -
Professora
This is used when noun in question is an object in sentence.
Video sam tvoga profesora (I saw your professor).
5. VOKATIV -
Profesore
Used when calling:
Dobar dan, profesore, kako ste? (Good afternoon, Professor, how are you?)
6. INSTRUMENTAL -
Profesorom
Used in frases such as:
Ići ću sa profesorom (I will go with professor).
7. LOKATIV -
Profesoru
This is very often iqual to Dativ. The diference is that is used with passive verbs, like when you are thinking of someone, writing about someone.
Jučer sam se sjetio mog starog profesora (Yesterday I remembered my old professor).
SV
About genders - I believe that genders in Croatian (like Serbian) are very logical (with very few exceptions).
1. Almost all words ending with
A are feminine: kuća (house), sestra (sister), ruka (hand).
2. Almost all words ending with
O and
E are neutral: jaje (egg), nebo (sky), drvo (tree), jedinstvo (unity).
3. All others are masculine.
brat (brother), volan (wheel), otac (father).
Exceptions are: feminine - stvar (thing), pomoć (help), etc.
)) 
