Quote Originally Posted by deker
As I notice there is wide using the genitive form of words in accusative case.
From yandex:
"Я не знал этой песни" - 253
"Я не знал эту песню" - 23
"Я не понял этого вопроса" - 308
"Я не понял этот вопрос" - 125
The most of Russians say these incorrect?
Genitive and accusative cases in sentences with a negation are often used interchangeably. They convey slightly different meanings and sometimes only one of the cases can be used: either genitive or accusative.

viewtopic.php?f=3&t=9041&p=105022&hilit=negation#p 105022
viewtopic.php?f=3&t=15314&p=191643&hilit=negation# p191643