A girl - en pige the girl - pigen girls - piger the girls - pigerne
In general most nouns get an -er or -r suffix in plural. And in definite plural most get -rne or -erne.
Some are irregular and change vowels. Like mand -> mænd in danish or woman -> women in English.
Those that do not get an "r" in plural, often don't get one in definite plural either, thus they just get -ene. E.g "mændene" - the men, "husene" - the houses.