Translation:My parents bought their house in the seventies.
sitt can point back at han, hon or de.
Jag köpte mitt hus.
Du köpte ditt hus.
Hon/han köpte sitt hus
Vi köpte vårt hus
Ni köpte ert hus
De köpte sitt hus.
Not if it's their own, but if it's some other people's house, sure. Like, That's the Smith family. My parents bought their house in the 70:s.
In Danish it would be "Mine forældre købte deres hus i halvfjerdserne". There seems to be a difference between Danish and Swedish regarding personal pronouns though. I didn't know about that difference but I found http://www.lardigsvenska.com/2010/11/personliga-promonen.html: "Den gyllene regeln: Sin/sitt/sina med objekt hans/hennes/deras med subjekt"
So you would say: "Deras hus är från sjuttiotalet" (which may or may not be true), but "De köpte sitt hus på sjuttiotalet".
I actually like the Swedish form better than the Danish. The Swedish plural form is more consistent with the singular form, and it's less ambiguous. For example in Danish you would say "Han købte sit hus" (not "Han købte hans hus", unless you're from Jutland :-) ) which is equivalent to the Swedish plural form.
[Edit, the following makes no sense, see Arnauti's comment below] Looking back on the Swedish and English sentences I would claim that "Mina föräldrar köpte deras hus på sjuttiotalet." should be accepted, because the English sentence is ambiguous (if you're a nitpicker). "My parents bought Peter and Mary's house" may or may not be the same as "They bought their house". In Swedish this ambiguity is resolved by using deras/sitt as subject/object.
Don't take my word for it, have a native confirm.
If we translate "Mina föräldrar köpte sitt hus på sjuttiotalet." to Danish it is "Mine forældre købte deres hus i halvfjerdserne" which is why it looked like an error for me to see "sitt" here instead of "deras".
I think duolingonaut and Arnauti should collaborate on making a course of Swedish for Danish speakers and vice versa! I am feeling mildly dyslexic trying to learn Swedish spelling already ;)
In this case you're supposed to translate from Swedish into English, so of course neither sitt nor deras hus is accepted, you must write their house. :P
I guess you're thinking of the reverse version of the sentence, and there both versions are accepted since the English is ambiguous.
So just to make sure, Danish doesn't use sin/sit in the third person plural and Sweidsh does? I'm really sorry but I'm tired and I am a bit confused with your discussion :D
Thank you. I felt very strange when I saw this sentence even though in my native language there is also a similar pronouon used for both singular and plural. Good to see I've retained some Danish thinking :)
I guess if it's not their house anymore it'd be Mina föräldrar köpte sig ett hus på 70-talet
I honestly can't really think of a good translation - at least one that's still relevant today. Obviously, every age has its niche words. Maybe fräsigt or something, that sounds really outdated and was in actual use at one time. But if I were to translate "groovy" today, I'd probably just stick to the English equivalent. :)