"They like it."
Translation:De tycker om det.
Yes, it's like using sie/er/es or elle/il as 'it'.
Common gender is 'common' to all nouns that used to be feminine or masculine historically, the other words are neuter ('it'). So words like flicka, man, kvinna, pojke are not feminine/masculine in Swedish, they are common gender.
These answers seem really inconsistent. Sometimes det is accepted, sometimes it isn't. Am I missing something?
Both det and den are accepted here, because this is a sentence where we're talking about a specific object, but we don't know what that object is. In many sentences, only det works because it is an 'empty' pronoun that does not refer to a specific object. (we usually compare with it in it is raining) I think it's pretty consistent in the course by now, there may be some errors left but there aren't huge amounts of them. Ask again on other sentences if there isn't already an explanation in the comments there.