Adj endings-masc/indef

Indefinite articles and possessive adjectives need to show the gender of the noun.
"Ein" does not tell us if it is masculine or neuter, so use "er" on the adjective to show it is masculine in the nominative and "en" for the other cases.
ein dummer Hund
einen dummen Hund
einem dummen Hund
eines dummen Hundes