"It is my sock."
Translation:Det er sokken min.
You can use "det" as a kind of unspecified pronoun (I don't know what the correct term is). I believe that in English, you can also say something like "It is my uncle", but if you keep speaking about him you would use "he". So the sentence above is correct, but if you keep speaking about your sock you will use "den". For instance: "Det er sokken min. Den er rosa."