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Czego, Czemu, Co, Kto, Jak, Dlaczego? [64]
I think the answer you are looking for is something like this:
kto/co? - who/what?
kogo/co?- who/what?
kogo/czego? - of whom/what?
komu/czemu? - to/for whom/to/for what?
kim/czym? -by means of who/by means of what?
o kim/o czym?- about who/what?
However to write out the cases like this is neither comprehensive nor terribly helpful. For example, (as you point out) it doesn't account for the occasions when
kogo/czego just means who/what nor does it clarify the difference between
kto/co and
kogo/co.
If I understand your question correctly, you are trying to find some unambiguous English translation of, say, kogo/czego in order to help you understand the meaning of
mamy/chleba [genitive of
mama (mum) and
chleb (bread) respectively. But it doesn't work like that. Just as
chleba can occur in various grammatical situations (
Chleba nie ma - there's no bread,
Potrzebujemy chleba - we need bread,
Kupiłem dużo chleba - I bought a lot of bread. (only the last example corresponds to the understanding of the genitive as meaning "of" something)), so it is with
czego (
Czego nie ma? - What isn't there?,
Czego potrzebujemy? - What do we need?,
Dużo czego kupiłem ? -What did I buy a lot of?)
What you've been told about how Polish children is true, but it is simply to avoid using technical words like
mianownik (nominative) and
dopełniacz (genitive). Unfortunately it's not a shortcut to learning which cases to use where.