"The crab drinks the water."
Translation:Krabben drikker vandet.
Because "en krabbe" is common gender, and "vand" has no gender. It would be "et vand" but that does not exist just like a water does not exist.
There is no rule to follow to figure out if a noun is common gender or no gender. You just have to remember them all. Most are commen gender.
Vand (Water) is a part of a group of nouns that cannot be counted. they do not exist as plural and are -t words. You can't put "et" in front of them. Other words in the same group are blod (blood) mel (flour) græs (grass) tøj (clothes) guld (gold) and a lot more
That's the one thing Duolingo doesn't explain very well. An easy way to tell the difference is to say both out loud and plug both en and et into the sentence. One of them almost always feels weird to day it out loud.
Does anyone else think of Mr. Krabbe from Spongebob Squarepants when they see any variation of Krabbe? His name is literally Mr. Crab!!! Hahahahaha. :D