Adjectives in the dative and genitive cases change in the exact same way when they come after der-word or ~ein-word. In both cases they take a ~en ending regardless of the gender or plurality of the noun they modify:

dem blauen Hund
the blue dog

dem blauen tiere
the blue animals

einer blauen katze
a blue cat

Things are however a little more complicated when the adjective is not preceded by an article. For the dative case, masculine and neutral singular nouns receive an ~em ending:

Rotem Hund
Red Dog

Rotem Tier
Red Animal

Feminine singular nouns receive an -er ending:

Roter Katze
Red Cat

And plural nouns of all genders receive an ~en ending:

Roten Hunden
Red Dogs


In the genitive case, if masculine and neutral singular nouns are not preceded by an article they receive an ~en ending:

Grünen Hund
Green Dog

Grünen Tier
Green Animal

Feminine singular nouns as well as masculine and neutral plural nouns receive an -er ending:

Grüner Katze
Green Cat

Grüner Hunden
Green Dogs