Noun declension – two-form determiners: ein and kein

A noun can be preceded not only by the definite but by the indefinite article as well, of course. The indefinite article also changes its forms in the four cases but somewhat differently from the definite article. In the nominative its forms are ein (masculine), eine (feminine), ein (neuter). So, the masculine and the neuter forms are the same. Thus, the indefinite article is a two-form determiner as it has two different forms in the nominative singular (ein and eine). That is where the declension of the definite article (three-form determiners) and that of the indefinite article (two-form determiners) differ. The indefinite article does not have plural forms, of course:

               masculine  feminine   neuter    plural

   Nominative    ein        eine      ein        -
   Accusative    einen      eine      ein        -
   Dative        einem      einer     einem      -
   Genitive      eines      einer     eines      -

E.g.:

Ein Mann kommt. ‘A man is coming.’ (nominative singular, masculine)
Ich sehe einen Mann. ‘I see a man.’ (accusative singular, masculine)
Ich gebe einem Mann einen Brief. ‘I give a letter to a man’ (‘to a man’ is dative singular, masculine)
Das Buch eines Kindes ‘the book of a child’ (‘of a child’ is genitive singular, neuter)

Another two-form determiner is the negative form of the indefinite article (kein, keine, kein) but it also has plural forms.

               masculine   feminine    neuter     plural

   Nominative    kein       keine      kein       keine
   Accusative    keinen     keine      kein       keine
   Dative        keinem     keiner     keinem     keinen
   Genitive      keines     keiner     keines     keiner

E.g.:

Kein Mann kommnt. ‘No man is coming.’
Ich habe keinen Bruder. ‘I have no brother.’ / ‘I haven’t a brother.’