It also depends on what youre referring to.. Jeg spiser ost og jeg liker den = I'm eating cheese and I like it (it = the cheese, because cheese is a masculine noun and "den" can only refer to it). While "Jeg spiser ost and jeg liker det = I'm eating cheese and I love it (it = eating, because "det" is neutrum and it can only refer to the act of eating).
"Det er vann." "Vann" is neuter. "The water" translates to "vannET."
Edit: Also, I think if something was not previously specified as being neuter or masc./fem., then you would just say, "det." If someone pointed to a book and said, "Hva er det?" you would respond, "Det er en bok." Then, if it was an interesting book, you would say, "Den er interessant," since you had just established that it was a masc. noun.
Someone please correct me if I'm wrong. Takk!
Edit #2: I just ran across this discussion in which Deliciae says, "When referring to a noun that is yet to be introduced, you default to "det". Once the noun has been mentioned, you use a pronoun that matches its gender and number to refer back to it."
In the spoken language (and in the audio as far as I can tell), people will often drop the ‘e’, so that the last word is pronounced “dn”. It is still pronounced as a separate syllable, and with ‘n’ as a syllabic consonant.
Many people will also drop the ‘d’, so that only the ‘n’ remains, which is then merged with the ‘r’ in “drikker”, forming the retroflex consonant ɳ, and it is no longer a separate syllable. This can only be done if there is no kind of emphasis on the word “den”.
If there is emphasis on “den”, it is pronounced fully, rhyming with “hen”.
What I've written here applies to dialects in the east of Norway, such as around Oslo.