Ich habe einen interessanten Film gesehen.
English: I saw an interesting film.
Adjective declension after an indefinite article in German means changing the ending of an adjective that comes after words like 'ein', 'kein', or possessive pronouns (mein, dein, etc.), depending on the gender, case, and number of the noun.
Use this when you have an indefinite article (ein, kein) or possessive pronoun (mein, dein, sein, etc.) before a noun and an adjective. The adjective's ending changes based on the gender (masculine, feminine, neuter), case (nominative, accusative, dative, genitive), and whether the noun is singular or plural.
Ich habe einen interessanten Film gesehen.
English: I saw an interesting film.
Sie kauft eine rote Jacke.
English: She buys a red jacket.
Wir wohnen in einem kleinen Haus.
English: We live in a small house.
Er sucht eine gute Idee.
English: He is looking for a good idea.