"The boy drinks it."
Translation:Gutten drikker den.
Seeing as this sentence does not specify what the object 'it' actually is, and there is no context, it can refer to either a neuter or a masculine/feminine noun. This means that we need to allow both forms:
det = it (n)
den = it (m/f)
This sentence leaves all options open:
If the boy was drinking water, 'vann' (n), then 'det' would be the corresponding pronoun.
If the boy was drinking milk, 'melk' (m/f), or soda, 'brus' (m), then the corresponding pronoun would be 'den'.
However, you can also refer to either as a 'glass of water', 'cup of milk', 'bottle of soda', and so on, which means that 'it' could then take on the grammatical gender of the vessel.
Finally, 'den' is technically always an option for this sentence, no matter the context, because it will always be a drink, which is a masculine noun in Norwegian: 'drikk'.