Take an apple, okay I think I know, but why einen Apfel?
"nehmen" is a transitive verb, meaning the following noun needs to take the accusative case. "Apfel" is masculine, so it's "ein Apfel" in the nominative but "einen Apfel" in the accusative.
Okay, I was wrong. I had no idea why. What makes it accusative? This is where I really struggle.
The Duo lesson says: Zeigen Sie mir die Speisekarte bitte. for the sentence Please show me the menu. Why "die" and not "der"? Or den or whatever, why does it not change?