Quote Originally Posted by deker
But what difference in pattern between "Я не понял этого вопроса" and "Я не понял этого человека."?
Я не понял этого вопроса - genitive
Я не понял этот вопрос - accusative.
"Вопрос" is an inanimate noun and you can easily tell apart genitive and accusative cases. "Человек" is animate masculine noun so the accusative case is equal to the genitive one. You can substitute a feminine noun instead to find out the case: "Я не понял эту женщину". While "Я не понял этой женщины" is also grammatically correct, it can be used in very limited contexts only. Compare the number of results for these two phrases in google. Therefore we can safely assume that we have the accusative case here.