"It is my sock."
Translation:Det är min strumpa.
No, it's an en-word. The short explanation is that the construction it/this/that is is referenced in neuter (det/detta/det här är) in Swedish, unless you've already named what you're referring to.
So, for instance, take these sentences:
- This is a book. It is good.
- Det här är en bok. Den är bra.
The first this is a new object, so it's in neuter. But for the it in the second sentence, we know to which it refers, and so it takes the den form.
For a slightly longer explanation, you can read my reply here as well: https://www.duolingo.com/comment/7214464