"The soup is expensive."
Translation:De soep is duur.
Because the adjective is in predicative position (i.e.: after a copulative verb -here, is)
Singular: Common gender nouns take De, neutral take Het. Plural: Everything takes De.
I have seen "duur" and "dure" both used, and so I thought since "soep" is a "de" word, it would get "dure", but it said the correct answer uses "duur"? Don't "de" words get the "e" ending?
That's when the adjective is placed before the noun.
When used predicatively, adjectives don't get the extra e at the end.
Kostbaar is Precious or Valuable. Duur is just Expensive. (Online dictionary.)