Well, not really a sin, but simply “sin”. It’s one of those fun “false friends”- words that look the same in both English and Swedish, but have different meanings. And we could pretty much leave it at that, but unfortunately, this Swedish “sin” can be a bit problematic for English speakers.
You see, it’s a possessive pronoun. Yes, another one, I know, I know… And it’s also of the “not found in English” variety of personal pronouns, which is precisely why it can be a pain to learn and use it correctly.
So, what is this “sin” exactly? It may mean “his” or “her” or “its” or “their” depending on the subject of the sentence. Because you see, it’s used when the third person possessive pronoun in the sentence refers to the subject of the same clause. Confused? OK, let’s take a look at how it works in real life.
- Åsa är gift med Jonas. – Åsa is married to Jonas.
And this lovely couple will now demonstrate when you use “sin” and when some other possessive pronoun is needed.
- Jonas besöker sin pappa ofta. – Jonas often visits his (own) dad.
- Åsa ringer hans pappa varje dag. – Åsa calls his (Jonas’s) dad every day.
See what I mean? In the first sentence, because Jonas is the subject and his dad is the object, instead of “his” we need to use “sin”. It means “his”, but more like “his own”. And that’s basically the gist of it. If you can’t stick “own” after the possessive pronoun in a sentence, then you must use “hans” or “hennes” or “deras”.
Now, Åsa will provide us with one more example.
- Åsa tvättar inte sin bil. – Åsa doesn’t wash her (own) car. (hehehe, she’s just like me!)
- Jonas älskar Åsa, så han tvättar hennes bil. – Jonas loves Åsa, and so he washes her car. (and Jonas is just like my husband, LOL!)
And that’s basically it. You just need to remember that “sin” can’t be used with the subject. For example:
- Jag känner Åsa. - I know Åsa.
- Hennes man arbetar på vårt kontor. – Her husband works at our office.
See? You can’t say “Sin man arbetar på vårt kontor”.
The easiest way to remember when to use “sin” and when to use a different possessive pronoun is to keep in mind these two simple sentences:
- Jonas älskar sin fru. – Jonas loves his (own wife).
- Jonas älskar hans fru. – Jonas loves his (some other guy’s) wife.
And of course, because it’s Swedish “sin” morphs into “sitt” when it is used with “ett” nouns and becomes “sina” when used with plural nouns.
Now, you see that it’s important to learn how to use “sin” correctly - it can help you stay out of trouble when talking about another man’s wife! 
Because otherwise, it’s a sin!
