"One of them sings."
Translation:En av dem synger.
'en' is the indefinite article 'a/an' for masculine nouns, 'ett' is the numeral 'one' for neuter nouns.
Norwegian has three genders and uses 'ei'(feminine), 'en'(masculine) and 'et'(neuter) as the indefinite article for these. For the numerals 'éi', 'én' and 'ett' are used, but these are often interchangeable with the indefinite article.