here is a great video that really helped me understand the cases of German. https://www.youtube.com/watch?v=8VowGvnRU_s&list=PLYzp2xhTw9W1Xfjp8_0oxsULeUw5g80TH&index=1
Because you are saying to someone else they are bringing the apple. If you were going to bring the apple, you would say in English, "I'm bringing the apple" whereas, as you are telling someone else to bring an apple, you say "You are bringing the apple". Hopefully this may help.
“denn” means “because”; “der” is “the”, but only for masculine nouns in the nominative case, feminine in the dative case, or feminine or plural in the genitive case; “den” is “the” for masculine nouns in the accusative case and plural nouns in the dative case.
Please pay attention to spelling, especially in the phrases you want translated. And don’t be afraid to use Google search. And for smaller, more versatile words such as the, to, like, and for, note that a word-by-word translation will not always work.