"The black shoes are mine."
Translation:De svarta skorna är mina.
"De" is used when the noun is plural and when there is an adjective before it, just like you have to use "den" and "det" in the singular when nouns have adjectives. For example, you have to say "Den vita boken," and not "Vit boken." As you can see, you still have to keep the noun in the definite form.
If it is singular you say: En vit sko (a white shoe) All three words are in indefinit form Den vita skon (the white shoe) Definite article+definite form of the adjectice+ definite form of the noun)
But in plural it is: Två svarta skor (two black shoes) De svarta skorna (the black shoes) Pretty advanced grammar for this level, I think!