"We are eating cheese and an egg."
Translation:Vi spiser ost og et egg.
Most likely because they didn't eat a whole cheese, just an unspecified amount of one.
"Vi spiser ost og egg" is a valid sentence, and a normal way of expressing it, but it's not the most faithful translation of the English sentence, which adds an indefinite article to "an egg", but not to "cheese" which is used in an uncountable sense.
Note that "(en) ost" is still indefinite, it's just lacking the indefinite article. "Osten" is the definite singular.
Yes, that's the rule, but "et egg" is a neuter noun. :)
en = indefinite article for masculine nouns.
ei = indefinite article for feminine* nouns.
et = indefinite article for neuter nouns.
*feminine nouns may be declined as if they were masculine, so they can take "en" in place of "ei".