The demonstrative adjectives "this," "these," "that," "those," and "what" are identical to the demonstrative pronouns, but are used as adjectives to modify nouns or noun phrases, as in the following sentences:

When the librarian tripped over that cord, she dropped a pile of books.

In this sentence, the demonstrative adjective "that" modifies the noun "cord" and the noun phrase "that cord" is the object of the preposition "over."

This apartment needs to be fumigated.

Here "this" modifies "apartment" and the noun phrase "this apartment" is the subject of the sentence.

Even though my friend preferred those plates, I bought these.

In the subordinate clause, "those" modifies "plates" and the noun phrase "those plates" is the object of the verb "preferred." In the independent clause, "these" is the direct object of the verb "bought."

Note that the relationship between a demonstrative adjective and a demonstrative pronoun is similar to the relationship between a possessive adjective and a possessive pronoun, or to that between a interrogative adjective and an interrogative pronoun.