This is the 8504th most frequent Russian word.
нину
"Нину" is the accusative or prepositional case of the name "Нина" (Nina), used to refer to a person with this name.
Here, 'ни' is used as a part of the expression 'ни одну' to emphasize the absence of a single mistake, demonstrating its grammatical role in a negative context.
Я не навидел ни одну ошибку в её работе.
I didn't notice a single mistake in her work.
In this sentence, 'Нину' is the proper name 'Nina' in its accusative case, functioning as the direct object of the verb 'заметили' (noticed).
На празднике Нину не заметили сразу.
At the celebration, Nina wasn't noticed immediately.
Here, 'Нину' is again the name 'Nina' in its accusative form, noting the person whom the action of seeing is directed toward in a first-person past experience.