6格和4格。

[从同呼]

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2017.04.15Johny so, in this case the Accusative case is used to refer to particular nouns and the Genitive case is used to modify non-particular nouns, right?

Yes, this is one of grammar ways to show theme-rheme relations (already known/new information) in the sentence. In Russian, word order and noun cases (sometimes plural number) is used, while in English you use articles and word order.

[从同呼]

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Johny,
1. I failed to see the wall (you had previously talked to me about)
2. стенЫ  I didn't see a wall (any wall)
стЕны I didn't see walls/the walls