"A cat and a dog."
Translation:En kat og en hund.
I've seen it from doulingo 'tips' page, and that's because 'kat' has a short vowel sound. (Danish mark the short vowel by doubling the consonant that follows the vowel. However, they don't do it when nothing follows it.) So here, 'a' from 'kat' is a short vowel, but they did not double the consonant 't' because there is nothing following it. However, when 'kat' is made plural, there is 'en' at the end so the 't' can be doubled. I'm not exactly sure if this is an accurate explanation, but you can always read about it in the 'tips' section. So I hope you will look at that. It is in the 'Animals' lesson.