In B-1 Norwegian, Lesson 111-120, there are two sentences which stand out:
“Jeg gir ham en ol” meaning “I give him a beer.”
BUT
“Olet er kaldt” meaning “the beer is cold.”
Nouns which have “en” as a prefix should have “en” as a suffix", while those which have “et” as a prefix should have “et” as a suffix, so “et ol” meaning “a beer” should be “olet” meaning the beer, not “olen,” as in the aforementioned lesson.