In the sentence, “Those are curtains,” “those” is a 

Pronouns are words utilised in place of nouns. They can only be used once the noun is used or mentioned. Pronouns are of several types, such as personal, reflexive, emphasising, relative, demonstrative, indefinite, interrogative, and possessive. Demonstrative adjectives qualify nouns and precede them, and emphasising pronouns are used for the sake of emphasis, the examples of which are “yourself,” “ourself,” and “myself.” Words that represent nouns or point out nouns are referred to as demonstrative pronouns. So, in the sentence, “Those are curtains,” “those” is a demonstrative pronoun.