Does "om" mean about?
when do I put Du or dig??
Du is used if you (singular, of course) is the subject of the sentence, while 'dig' is used when 'you' is the object of the sentence.
Is that 'if you are the subject of the sentence'? XD
I think is that this sentence is a bit too long to register.
Why is "jeg fortæller dig om DEN" marked as incorrect?
It should be marked as correct, since both den and det mean 'it'. I don't know why it's marked as incorrect, maybe it's an error made by duolingo? I'm not a native speaker though, so if I am wrong, someone correct me.
That seems to happen to me a lot too
You only really use "den" in Danish if it is clear what you are talking about. "HundEN er sød. Jeg fortæller dig om DEN"