"He eats the apple."
Translation:Han spiser æblet.
Ooh! I can answer this one! Danish uses suffixes for "the" rather than a separate word. et æble = an apple, but æblet = the apple. So when you say "et æblet", you're saying "a the apple," which makes no sense. As for spise, that's just an incorrect conjugation. All verbs take -r in the present tense, so it's "spiser".
The same reason you don't say "they the apple" in English. "De" can only translate to "the" when there is an adjective between the noun and definite article in English and the noun is plural. So:
Han spiser æblet. = He eats the apple.
Han spiser det røde æble. = He eats the red apple.
Han spiser guleroden. = He eats the carrot.
Han spiser den lilla gulerod. = He is eating the purple carrot.
Han spiser æblerne. = He is eating the apples.
Han spiser de røde æbler. = He is eating the red apples.
Han spiser gulerødderne = He is eating the carrots.
Han spiser de lilla gulerødder. = He is eating the purple carrots.
(I mentioned carrot(s) to show that "de" is used with both grammatical genders)