Scott and KidK, thanks both of you...
Yes... it's tough isn't it. Numbers apart, can you believe that the whole thing works differently depending on the gender of the word... !
I guess it's a matter of "slowly does it" and "every little helps".

But I don't know in which end to approach the kasus-es.

-It's impossible to simply memorize all of it.
-And it's so complicated that you couldn't (as an adult) learn it simply by listening or reading (or do you think it's possible?)
-And in addition to the very complicated rules, there are large numbers of exceptions.

It's very overwhelming...