When exploring a new place—like a vibrant coral reef—you often want to point out what you see. To state that something exists, Danish uses the phrase der er (there is / there are).
These existential sentences follow a strict rule called the definiteness effect (definithedseffekten). This means that the subject following der er must always be indefinite. Because you are introducing new, unknown information to the listener, it must take an indefinite form (such as using en, et, or an indefinite plural).
| There is a tropical coral reef down there. | Der er et tropisk koralrev dernede. |
| There is a large predatory fish down here. | Der er en stor rovfisk hernede. |
This rule applies to plural nouns just as strictly. Notice how mange anemoner (many anemones) is indefinite:
| There are many anemones by the yellow submarine. | Der er mange anemoner ved den gule ubåd. |
Talking About Specific Things
Once an object is known, it becomes definite (e.g., "a coral reef" becomes "the coral reef"). Because of the definiteness effect, we can no longer use der er to describe it.
If you want to talk about a specific, definite thing, it simply becomes the regular subject of a standard sentence instead:
| The tropical coral reef is very big. | Det tropiske koralrev er meget stort. |
| The coral and the anemone are both red and green. | Koralen og anemonen er både røde og grønne. |
Remember that the definiteness effect strictly applies to der er constructions. For regular action verbs, you can freely mix and match definite and indefinite nouns:
| The diver sees a coral grow on the seabed. | Dykkeren ser en koral vokse på havbunden. |
| The woman sees the diver swim toward the submarine. | Kvinden ser dykkeren svømme mod ubåden. |