Questions & Answers about Det stora huset är deras.
Why does this sentence use both Det and the -et ending on huset?
This is a rule in Swedish often called double definiteness. Whenever you have an adjective describing a specific, definite noun, you must use both the freestanding definite article (Det for ett-words) and the definite ending on the noun itself (-et).
Since hus is an ett-word, shouldn't the adjective be stort? Why is it stora?
Because the noun is in the definite form (meaning "the big house" rather than "a big house"), the adjective must also take its definite form. For almost all Swedish adjectives, the definite form just adds an -a to the end, regardless of whether the noun is an en-word or an ett-word.
Does the word deras need to change its spelling to agree with the ett-word huset?
No! Unlike words like min (my/mine) which change to mitt for ett-words, deras is fixed. It stays exactly the same whether it describes an en-word, an ett-word, or a plural noun.
Is there a difference in Swedish between "their" and "theirs"?
No, Swedish uses the exact same word, deras, for both. You use it to say "their house" (deras hus) and "the house is theirs" (huset är deras).