An adjective placed predicatively agrees with the noun it refers to in number and gender.
Per er uhøflig. (Per is impolite)
De er uhøflige . (They are impolite) The adjective gets an “e” because “de” are plural form.
Rommet er tomt (The room is empty) The adjective gets a “t” because “rom” is a neutral noun.
Rommene er tomme . (The rooms are empty) The adjective gets an “e” because “rommene” are plural form.