"It is a cow."

Translation: Det er en ku.

[kokiri85]

It marked me as wrong for using "ei," but I thought you could use either "ei" or "en" for the feminine ones?

[alek_d]

"Det er ei ku." is accepted. We've had some reports that Duolingo marks answers with "ei" as a typo recently, even when it is one of the accepted answers in the course.

You're right that you can use either "ei" or "en" for "ku".