"The man is eating an apple."
Translation:Der Mann isst einen Apfel.
Just thought of this potential "Eselsbrücke" (donkey bridge a.k.a. mnemonic aid).
Remember Kennedy's "Ich bin ein Berliner!" = "I am a Berliner!"? "ein Berliner" is the nominative object, i.e. the noun or subject "Berliner" is in its basic form (as we memorize nouns). Just add "Ich sehe einen Berliner!" = "I see a Berliner!" to it. Here, "einen Berliner" is the accusative object, its article is changing, "ich" is the nominative object acting upon the accusative object. Now you will memorize that "ein" is used for masculine nominative objects and "einen" for masculine accusative objects.