This is even difficult for romanians... Beleive me! :P
But take two examples! There are more, but I dont want to confuse you:
1.Nu e niciuna acolo. (There is none there....-NICIUNA -negative pronoun)
2. Nu-mi place nici una, nici cealalta. (I don"t like neither one nor the other...-NICI-conjunction; -UNA-indefinite pronoun)
Is it clear? :)
Niciuna, nici una: feminin form, sg
Niciunul, nici unul: masculin form, sg