"The bear eats a sandwich."
Translation:De beer eet een boterham.
This depends on the gender. There is only one gender where Dutch uses "het" as the definite article: For a singular noun with neuter grammatical gender. Only 25% of the words are neuter, but unfortunately, there's no rule that will always tell you whether a noun is neuter.
There are, however, some cases where you can expect a noun to be neuter:
When a word is a diminuitive, it will always be neuter, and thus get "het": "het bootje": the boatlet / the small boat; "het blaadje": the leaflet / the small leaf; "het meisje": the maidlet / the small or young maid or maiden.
Likewise, the noun form of a verb is neuter: "het leren" (the learning), "het vergeten" (the forgetting). Games and sports, even when not the noun form of a verb, are all neuter: "het dammen" (draughts), "het tennis"
Names of languages: "Het Engels" (English), "Het Nederlands" (Dutch), of metals: "het goud" (the gold), "het blik" (the tin), and of directions: "Het Westen" (the West), but not the area so indicated "de Oost" (the East).
Words with the suffix "-isme": "het cynisme" (the cynicism), or with the suffix "-iment": "het experiment" (the experiment).
Two-syllable words with a prefix "be-", "ge-", "ont-" or "ver-." (Well, really one-syllable words with these prefixes in front of them.)
This leaves a group of mostly old one syllable words that are also neuter, but that no definite rule exists for: "het huis" (the house), "het boek" (the book), "het paard" (the horse).
To learn, make sure that for every noun you learn, you memorise it with the article if it uses "het". (There's not much reason to also memorise which words take "de", as that's the other 75% of all nouns.)
If you don't know, this website might: < https://www.welklidwoord.nl/ >.
And if I'm not mistaken if the animal name specifically implies a male or female (so the biological gender is clear) it is always de
So de leeuwin
(Lioness, female bear, tomcat, mare, male dog, bull, stallion)
(interesting that bear and dog does not have a common word for it in english)