Getting down to 'cases'  (with Ath. 2b) 

You've already dealt with the so-called nominative and accusative
--the endings for subject (-os) and object  (-on)of the verb. 


The two middle cases can be confusing in the same way, 
because they have a basic meaning with or without a preposition.
 
The cases,  'genitive' and 'dative' convey the basic relations 
"of" or 'belonging to' (possession, etc,); and "to/for" (=indirect object).