Learn Swedish: De säljer sina bilar. - They sell their own cars.

In Swedish, when a third-person subject like 'they' does something to their own things, you use the reflexive form (sin, sitt, or sina). If the sentence used deras, it would mean they are selling someone else's cars. Because the subject De owns the cars being sold, we use sina.

Why do we use the form sina instead of sin or sitt?

You choose between sin, sitt, and sina based on the noun being owned, not the owner. Because bilar (cars) is plural, we use the plural form sina. If they were selling just one 'en' word (like a computer), it would be sin, and for one 'ett' word (like an apple), it would be sitt.

Why does sina use the indefinite form bilar instead of the definite bilarna?

After any possessive pronoun (like min, din, or sina), Swedish requires the indefinite form of the noun. Even though we are talking about specific cars, the possessive word already makes it definite enough. It works just like English: we say 'their cars', not 'their the cars'.

I sometimes see 'they' spelled as 'dom'. Why is it De in this sentence?