"The sheep eats something."
Translation:Het schaap eet iets.
Because "schaap" is a "het"-noun, not a "de"-noun. There is no deeper rule here; you just have to know which nouns use which article.
But in case you know German, "het" goes with cognates of German neuter nouns ("das Schaaf" in this case), while "de" goes with cognates of German masculine and feminine nouns. (There are some other, more obscure languages that you could use in place of German here.) Also, if a noun has a suffix, then the suffix is all that matters. Finally, most nouns use "de", especially those for people (but not all of them). Those are all of the rules that I know!