Croatian has 7 cases. Each of them replies on specific questions. About questions you can read here: http://learn-croatian.com/grammar-cases.php Usually we put questions for instrumental "with whom", "with what" but often instrumental comes without "with" which is translated like "s" or "sa". Let's see some examples (for instrumental): We write with pencil. (We write WITH WHAT (with pencil)) - Pišemo OLOVKOM. Olovka - singular, feminime, nominativ --> s, f, N I come with friend (I come WITH WHOM (friend) - Dolazim S PRIJATELJEM. Prijatelj - s, m, N Prijateljica - s, f, N Without preposition "s/sa", instrumental can be: 1. adverb of place Hodam ULICOM (I am walking by street) ulica - s, f, N 2. adverb of time Ne radimo NEDJELJOM (We don't work on Sunday) nedjelja - s, f, N 3. attribute Promet se odvijao ŽELJEZNICOM (Traffic went by railway) željeznica - s, f, N and so on... I don't see any easy way to learn cases. Comparing them? I'm not sure what to answer on this because every case has function for himself. Here is declination of girl's name Lara: Nominativ: I am LARA Genitiv: There isn't LARE Dativ: I'm taking this to LARI Akuzativ: I see LARU Vokativ: Hey, LARA (or LARO) - mostly depends on different parts of Croatia Lokativ: I'm talking about LARI Instrumental: I'm coming/going with LAROM. You should have in mind that this is example with feminime noun (in singular) which in nominative ends with vowel. Endings are not the same with feminime nouns which in nominative form don't end with vowel. Example with noun Ines (girl's name which doesn't end with vowel): N: I am INES G: There isn't INES D: I'm taking this to INES A: I see INES V: Hey, INES L: I'm talking about INES I: I'm coming/going with INES Also, there are musculine form and neuter, both with forms that end with vowel and without them. Plus singular and plural.