"I like it."
Translation:Jeg liker den.
It could be both den and det here. "Den" is a replacement for a masculine word.
e.g. talking about a dog (en hund, masculine)
Jeg liker den. Jeg liker hunden. Jeg liker den hunden. - I like it. I like the dog. I like that dog.
Talking about a tree (et tre, neuter)
Jeg liker det. Jeg liker treet. Jeg liker det treet. - I like it. I like the tree. I like that tree
If it is not replacing a word, it is by deafult "det"
Neuter ones as well as being used when it introduces nouns (when it doesn't replace any specific noun).
Give this exercise a try (click on "Det, den eller de?"): http://www3.hf.uio.no/iln/studier/evu/norskkurs/igin/niva3/Pronomen
Edit: reworded my sentence to make it more clear.