What are the n-word and t-word in the danish language

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


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


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.