Correct IPA pronunciation for barn is
/bɑːɳ/ and for barnet is
ei jente - jenta - jenter - jentene
et barn - barnet - barn - barna
As for all feminine nouns, you have the option as treating "ei jente" as masculine:
en jente - jenten - jenter - jentene
If you use the desktop version of Duolingo, there will be grammar explanations available just below the lesson overview when you open a grammar skill.
Bonus tip: In English, you conjugate a verb, and decline a noun or an adjective. In Norwegian, you can use the verb "å bøye" for either action.
"Although the t is pronounced as such in the phrase et barn, it turns silent in the definite form, barnet, which is pronounced more like barneh. This is the case with all neuter nouns in the singular definite form. Be sure to drop the t sound, otherwise you might sound rather Swedish." You can find a few tips and grammatical rules in 'Tips and Notes' section under every lesson. :)
Preposterous! Everyone knows that on days in which the wind blows from the northeast, one must use the object-verb-subject form- unless (of course) that day also falls in between the second and third weeks of a spring with three full moons- OR, if on that day, the speaker was wearing socks longer than three inches. It's a common mistake; don't feel bad.