Adjektivdeklination: after ein/eine (indefinite article) – Telc B1

After ein, eine, kein, etc.: nominative masculine takes -er (ein guter Tag), nominative feminine -e (eine gute Idee), nominative neuter -es (ein gutes Buch). Accusative masculine takes -en (einen guten Tag). The rest often -en. So the adjective "fills in" the case/gender information that the article doesn't show (e.g. einen = acc. masc., so the adjective has -en).