I'm not a native Norwegian speaker, but I believe that is because this sentence is essentially equivalent to "A cat is at our house," focusing on the cat's existence or presence.
der is used to emphasize the location of something or someone, like you're pointing your finger in that direction and saying "There it is! It's over there!"
If you can replace "there" with "here" (
her) and produce a sentence that still makes sense, then using
der is probably appropriate in that case:
Det er mat på bordet = There is food on the table. (edited, thanks HakonSoreide!)
("Here is food on the table" doesn't really make sense.)
Der er maten! = There's the food!
("Here's the food!" makes sense.)
Native speaker here, and I am sorry to say it's not quite as simple as that.
Also "det er maten på bordet" means "that is the food on the table". If you want to say "there is food on the table" it should be "det er mat på bordet."
English has two dummy subjects, "it" and "there", and Norwegian also used to have two, "det" and "der".
"Der er mat på bordet" is correct Norwegian, though quite old-fashioned now, and means "there is food on the table".
If you want to use the demonstrative "there", the sentence becomes "Der er maten på bordet", meaning either "now the food is on the table" or "the food is there on the table".
The demonstrative "der" is always stressed when speaking, but dummy subject "der" and "it" are never stressed.
You ask an interesting question. In fact, Norwegian used to distinguish between the two different dummy subjects it is and there is same as in English with det er and der er respectively.
Der er as a dummy subject is now considered archaic, and is seldom used. If you see Der er the der is then most likely the demonstrative there rather than the dummy subject there.
Yes, it gets rather confusing when demonstratives and dummy subjects look exactly the same...
Dummy subject "der er" can be rewritten as "der finnes", "der lever", "der eksisterer". In other words, it talks about something existing, something concrete, often something living, that can thus have a location in space and time (even though it is a dummy subject, it is still closely linked to its etymological origin as a demonstrative).
Dummy subject "det er" is now used for both cases, but when it means "it is", just like in English, what you are talking about it usually something abstract, or what in other languages might be a subjectless verb describing a state, such as
- "Det er varmt ute" = "The weather is hot".
- "Det er sommer" = "It is summer".
Hus is a noun, huse is a verb, and hos is an adverb or preposition. They are all etymologically derived from the word «Hus».
- Vi huser katten.
- Katten er i huset vårt.
- Katten er hos oss.
These don’t mean exactly the same thing, but they almost do, using the verb, noun and preposition respectively.
Norwegians don’t even think of hos and hus as being sort of the same word as they are used so differently.
One could say that, but it means something else: it means the cat is either on the outside wall or the roof. Also, "hos oss" doesn't necessarily mean at someone's house, though that is a common possible translation. If you were at someone else's house, you could say "hos oss". If you were in a restaurant and a cat suddenly came and stayed at your table, you could say it, etc. It only implies a clearly defined relative location.