In English and in German we have the “indefinite article”, or “der unbestimmte Artikel”. 
In English, the indefinite article is either “a” or “an”, depending on the spelling of the noun. In German, the indefinite article changes based on the three genders. In the nominative case they are: ein (masculine), eine (feminine), and ein (neuter). The indefinite article is used when talking about a non-specific noun and definitions, when talking about something we lack information about, or when referring to something for the first time.

There are several other words that follow the same declension as the indefinite article ‘ein’ : The negative article “kein”, and the possessive adjectives “mein, dein, sein, etc”. 

The endings of these ‘ein-Wörter’ are listed below along with the indefinite articles :

1. The Indefinite Article in different cases: 

Below is a chart for how the indefinite article changes based on gender and case. Notice that we do not use the indefinite article to refer to plural nouns. 

Examples: 
Das ist ein Hund. This is a dog. (singular, masculine, nominative)
Ich habe einen Hund. I have a dog. (singular, masculine, accusative)

2. The negative article

The endings of other ‘ein-Wörter’, such as the negative article “kein” are all identical to the indefinite article and its endings. Unlike the indefinite article, we can use the negative article to refer to plural nouns. 

Examples:
Das sind keine Rosen. These are no Roses. (plural, feminine, nominative)
Ich habe keine Haustiere. I have no pets. (plural, feminine, accusative)