Why not "fromagxon?"
The accusative ending -n ONLY applies to the direct object. And NEVER to nouns after prepositions :)
But why does there have to be a preposition there? Also, is there no -n on formaĝo because the amount of cheese being eaten isn't clearly defined?
What does da mean?
A prepositional OF to express an indefinite/some (iom) quantity of(da) cheese (fromagxo)
If we were saying "some of the cheese" we would change to "de la", since one never says "da la", correct?