"They drink water."
Translation:De dricker vatten.
No! The correct way of writing is "de", even though many people often only write "dom" in less formal texts such as blogs, SMS, e-mails and so on. You should try to learn the difference between "de" and "dem" and use them when writing.
If you are not sure, then "dom" could work, but it does not look as good.
Personally, I would never write "dom", not even in a SMS, but that's probably because I like to write properly. I also dislike "mej", "dej" and "sej" for "mig", "dig" and "sig" even though these spellings are accepted in general.
You use "den" in place of "it" when the "it" is used to refer to a specific noun that belongs to the common gender:
"I have a dog and it is cute." "Jag har en hund och den är söt."
"My computer won't start and I don't know what's wrong with it." "Min dator startar inte och jag vet inte vad som är fel med den."
"Death takes everyone in the end. It is unavoidable." "Döden tar alla till slut. Den är oundviklig.
You use "det" in the same way to refer to nouns that are of the neuter gender. You also use "det" for the abstract dummy "it", when it doesn't refer to any particular noun:
"It is raining" "Det regnar"
"It is going to get easier" "Det kommer att bli lättare"
"It is never a good idea to drink and drive" "Det är aldrig en bra idé att dricka och köra bil"