"A mouse is eating a sandwich."
Translation:En mus spiser en sandwich.
"En" and "et" does not correspond to the English articles "a" and "an." Danish nouns have two genders, and the indefinite article ("en" or "et") depends on this. If it is an en-word, then the article "en" is used. If it is an et-word, then the article "et" is used. You have to memorise the gender as you learn the word because there are rules later on which will also depend on their respective genders.
English does not have this, so most learners are thrown off when they start learning the language (at least I was).