"The boys eat an orange."
Translation:Drengene spiser en appelsin.
en is the indefinite article for common gender words (for example: en bil (a car), en seng (a bed), en appelsin (an orange))
et is the indefinite article for neuter gender words (for example: et hus (a house), et æg (an egg), et fjernsyn (a television))
Here is a good post from a Duolingo user. Unfortunately the "rules" for determining the gender of words (other than through some endings, and even then it's a bit unreliable) is highly inconsistent but 75% of nouns are common gender