If the noun is the common gender it will be “-en” in the definite singular: en bil, bilen (a car, the car) -er can never be used in the definite form. It is one of two possible endings (-er and -e) in the indefinite plural if the noun has a regular plural. “Biler” for instance means cars, not “the cars”. Inflecting nouns in Danish is unfortunately a quite unpredictable thing even for regular nouns. And many nouns have irregular plurals, so you have to learn the inflection for each word, for instance: et barn, børn (a child, children), en mand, mænd (a man, men). If you are unsure about inflections of nouns it is explained here: https://en.wikipedia.org/wiki/Danish_grammar#Nouns

"er" or sometimes "e" indicates that the noun is plural, that there is more than one, while "en" means "the", indicating a singular, specific noun. when you put the two together to make "erne," this means you're identifying a specific plural noun. example: building = bygning. buildings = bygninger. the building = bygningen. bygningerne = the buildings.