Ich habe einen roten Apfel.
English: I have a red apple.
Adjektivdeklination means changing the ending of an adjective in German, depending on the noun's gender, number, and case, as well as the article before it.
Use adjective endings when an adjective comes before a noun in German. The ending changes depending on the article (der, ein, etc.), the gender (masculine, feminine, neuter), the number (singular, plural), and the case (nominative, accusative, dative, genitive).
Ich habe einen roten Apfel.
English: I have a red apple.
Sie trägt ein schönes Kleid.
English: She is wearing a beautiful dress.
Wir wohnen in einer kleinen Wohnung.
English: We live in a small apartment.
Er spricht mit dem netten Lehrer.
English: He speaks with the nice teacher.