Ich sehe einen alten Hund.
English: I see an old dog.
Adjective declension after an indefinite article in German means changing the ending of an adjective (like 'klein', 'schön') when it comes after 'ein', 'eine', or 'kein'. The ending depends on the gender, case, and number of the noun.
Use this when you have an adjective right after an indefinite article ('ein', 'eine', 'kein') and before a noun. The adjective needs a special ending to match the gender (masculine, feminine, neuter), case (nominative, accusative, dative), and number (singular, plural) of the noun.
Ich sehe einen alten Hund.
English: I see an old dog.
Sie hat eine neue Tasche.
English: She has a new bag.
Wir kaufen ein großes Auto.
English: We are buying a big car.
Er liest keine langen Bücher.
English: He does not read long books.