"We have a wonderful son."
Translation:Mamy wspaniałego syna.
Why is this genitive (wspaniałego), rather than accusative (wspaniały)? Shouldn't genitive be reserved for a sentence talking about 'our neighbour's wonderful son' ?
This might sound strange to you, but this is indeed accusative. "Syn" belongs to the category "maculine animate" where the accusative and genitive forms are identical.
But masculine Accusative is the only case when it matters whether the noun is animate or inanimate. Animate looks like Genitive, inanimate looks like Nominative.
"Mamy wspaniałego syna". "Mamy wspaniały stół".