der Apfel or den Apfel
Which one is right? Or in each case should I use one instead of another?
Depends on the grammatical case. der Apfel is "nominative": Der Apfel ist rot. Apfel is the subject of the sentence here. den Apfel is "accusative": Ich pflücke den Apfel. Apfel is the object. What is being done with the apple? it is picked.
And just to do your head in, there are 2 more cases: Genitiv and Dativ, and all change with the Gender of the noun.
Its not a "plural/singular" thing, but rather a "subject/direct object" thing.
When it's plural AND nominative (nominative meaning, subject) you'd say Die Äpfel sind etcetc. if it's singular and it's the subject you'd just say Der Apfel ist rot, the subject of the sentence is Der Apfel.
If you say Ich esse den Apfel, Ich is the subject, and Apfel would be the direct object, and in this case (accusative) der changes to den.