According to the danish language’s grammar, the noun words (Substantiver/Navneord) can be either n-word or t-word. For example

T-Words

i.) et apotek. ii.) et glass iii.) et år

N-Words

i.) en bil ii.) en fisk ii.) en pige

So we can see that a prefix (en/et) of an indefinite singular word defines the type of the noun word.