Plural indefinite articles
Many charts I see look like this:
Nominative Indefinite Articles M F N PL ein, eine, ein, keine
My question is what to say if there is a plural? Would it be feminine? like the Nominative definite article?
There is no plural indefinite article in German, just like in English:
I eat apples = Ich esse Äpfel
However the negative article kein, which is essentially k + ein, does have plural forms:
ich esse keine Äpfel = i don't eat apples
Would it be feminine? like the Nominative definite article?
You shouldn't think of the plural article as a feminine article, and nouns in plural do not become feminine: it's simply that they happen to look the same, just like "sie" (she) and "sie" (they) look the same, but they're not the same, since adjective and verbs will agree differently with them.
They also have a slightly different declension:
Nominative: die (feminine) / die (plural)
Accusative: die / die
Genitive: der / der
Dative: der / den
There are two types of determiners, der-words, which decline like the definite article, and ein-words, which decline like the indefinite article. Because ein has only singular forms, it can't serve as a prototype for ein-words with a plural, so people often refer to ein/kein.
Noun gender determines how parts of the noun phrase (determiners, adjectives, nouns) decline in the singular. In that sense, nouns lose their gender in the plural, because the declension is the same regardless of gender.