Back to (hopefully) get things right, because one month ago me was slightly incorrect. When there's an adjective before the noun in definite form, den and det must be used and the adjective takes the plural form. If the adjective is later in the sentence, it is in singular form and is either en or ett form based on the gender of the noun.
If it's like German, I think it's that "svart" uses the ett style because it's at the end of the sentence as opposed to in front of the noun. At that point, if I'm correct, all the adjectives take an ett form. Google Translate doesn't seem to agree with me, though. I don't see this in the Adj 1 or Adj 2 explanation sections, so if it's a special rule, perhaps it should be added there.
When you use an adjective together with the definitive form of a noun you generally need the definitive article den/det before the adjective.
- "The wolf eats the duck." - "Vargen äter ankan"
- "I eat the read apple." - "Jag äter det röda äpplet."
- "I eat the apple." - "Jag äter äpplet."
There are a few exceptions where den/det may be left out.
- "Jag tar stora vägen till staden." - "I take the large road to the city." (It is till right to say "Jag tar den stora vägen till staden" in this case.
- Jag tar gröna linjen. - "I take the green line."
gramphos already commented about this, but the reason we use "den/det" is because there is an adjective before the definite noun.
"Vargen äter ankan" = The wolf eats the duck (no adj.).
"Vargen äter den svarta ankan" = The wolf eats the black duck (adj. before def. noun).
Hope this helps!