German adjectives change their form slightly to “agree” with the noun they modify. The gender, case (grammatical function) and number (singular or plural) are the things that cause changes in adjective form.
The only thing that happens to the adjective is that the ending adds or drops a letter. You can consult the chart at the end of the article for specific examples.
Adjectives come before the noun and, as in English, there are two basic ways of setting up the adjective-noun relationship:
1. Predicative adjectives: The noun is adjective.
Die Blumen sind blau.
The flowers are blue.
2. Attributive adjectives: An adjective noun.
Eine rote Blume.
A red flower.
Only the attributive adjectives get declined for gender, number, and case.
Das Museum ist alt
The museum is old.
Das ist ein altes Museum.
That is an old museum.
Wir sehen Kunst in dem alten Museum.
We see art in the old museum.