I just read from a latter question that both "skilpadden" and "skilpadda" are accepted (but can't go back to comment there so adding an additional comment here). However, if the course is intended to teach "skilpadda," as a feminine word, then in other questions where it's referring to "a turtle", shouldn't it be "ei skilpadde" instead of "en skilpadde"?
Thanks in advance for clarification :)
It confused me too. It all depends on the gender of the singular noun. It looks like this:
Singular: en skilpadde - Plural: skilpadden
Singular: ei skilpadde - Plural: skilpadda
It seems duo decided to teach the feminine version in plural, although they omit the usage of "ei" article in the singular case. So "en" in every case of "en" and "ei", but "-a" in almost every case of feminine noun. :)
(It says so in the tips and note section of the basics lesson, that there are particular nouns that Norwegian people use the feminine from of, (like "jente", which is "jenta" in plural), but they stick to using the masculine article nevertheless. In other cases (as I see, most of them), they remain using the masculine gender in plural.)