"The boy eats like a bear."
Translation:Drengen spiser ligesom en bjørn.
You could use "som" in this case, where it translates to "like".
But "som" can also be used to refer back to a subject already mentioned, "Kan du se bilen, som er rød?", "Do you see the car that is red". In this case both the words "som" and "der" can be used where "that" is used in the English sentence, and they are virtually interchangeable. I think that my Danish teacher tried to teach us, to only use "der" when referring to people, but I don't think that rule is used any more.
However "der" can also mean "there", as in "Bogen er der"/"The book is there", "Er der mere øl?"/"Is there any more beer?", and a few different uses "Hvad er der i vejen?"/"What's wrong".
There is a rule with der as a relative pronoun where you can't swap out som for it: der can be only used as the subject of the relative clause. Examples:
- Ser du manden, der/som gå ind i bygningen? - Som is the subject here, so you can interchange it with der.
- Det er ikke brødet, som vi spiser. - Vi is the subject in the relative clause, so som is an object. It can't be replaced by der here.