It's not precisely genitive of negation, but there is a tendency to put direct objects into the genitive case when a transitive verb is negated (particularly when the direct object is abstract).
I was wondering about the genitive case here as well. I found an explanation here: http://blogs.transparent.com/russian/accusative-and-genitive-in-negative-russian-sentences/
thanks a lot! I will remember this:
" We prefer to use accusative for tangible and genitive for abstract nouns. But we don’t always do. That is why you are as likely to encounter both"
I've done a small bit of research on the topic of negation in Russian, and it seems to be a really difficult topic, when you get away from the obvious нет [genitive]. I like Peatsickle's explanation about direct objects of negated verbs being cast in genitive, but I'm not sure if that can be a "rule". It seems more complicated than that.
Actually, извини is less formal, and more of an abbreviation (think "sorry"), while извините is more formal (think "my apologies") and is used with adults and strangers.
It's not really an abbreviation. It's that "извини" is used with people with whom you are on "ты" terms (close friends, relatives, children), and "извините" with everyone else.
"Pardon, i don't know the answer" fails with correction to "Sorry, i don't know the answer"
Pardon is more formal and would probably be translated as извините, but report it anyway because it's still an acceptable answer.
In other responses i have used "pardon me" for "извини" but here it is not accepted.