"They are girls."
Translation:De er jenter.
Det = neuter singular
Den = feminine or masculine singular
'Det' is also used in general statements, much like 'it' in English:
"It is raining."
So whenever you'd use 'it' in English, without 'it' actually replacing a noun in the sentence, 'det' is what you'll want to use in Norwegian.
What's the difference between jenta and jenter? I keep making that mistake...
dere = you (plural)
de = they, the (before a definite plural noun modified by an adjective)