Quote Originally Posted by deker
That's not only in negation. Likely a half of people say "я купил хлеба"
And this is not quite right to say that the genitive case is used instead of accusative, the sentences like "Я не понял этого вопроса" as a matter of fact is still in the accusative case but the form of words is the same with genitive case which similar to the declination of animate nouns.
And the sentences like "У меня есть машина" & "У меня нет машины" is different matter, I don't know how it is worded.
It is genitive case. I can't see a point. Why do you think it is accusative?