"It is a duck."
Translation:Det är en anka.
Because you use "det" when you are talking about something as yet undefined, which "it" is in that sentence. You don't define it as a duck--and therefore having a common gender--until the end of the sentence.
If you were to continue the conversation, say by going "It is a duck. It is brown." Then since you've established in the context of the conversation that "it" means "duck" you would use den instead.
Could you come up with an example using Den? Det ar en anka, Den ar brun anka. Is it right?