"I do not know our neighbor."
Translation:Я не знаю нашего соседа.
I don't fully understand the нашего yet. I think I read it in the tips & notes but I can't find it. :/
"Нашего" is "наш" in accusative case. The possessive pronoun has to agree with the noun in Russian and in this sentence "сосед" is in the accusative case, so therefore "наш" must also be in the accusative case.