A relative adjective is basically a relative pronoun that is used within a sentence as an adjective, ultimately modifying or describing a noun or pronoun. For example, the word “that” is a relative pronoun and is often used in a sentence like “I have a cat that eats a lot.” In this sentence, the word “that” is used as a pronoun in the place of “cat” to indicate what object in the sentence “eats a lot.” If the relative pronoun “that” is used in an adjectival form, such as in the sentence “Hand me that book,” it is a relative adjective as it describes the noun “book.”
One of the easiest ways to understand the purpose of a relative adjective is to first understand relative pronouns. These are words that function like other pronouns, by taking the place of a noun in a sentence, but do so in reference to a noun already within that sentence. They are “relative” in that they cannot operate by themselves and still express any type of clear meaning. A pronoun like “he” still has some general meaning outside of a specific context, while a pronoun like “that” is essentially meaningless without the noun to which it refers.
In a sentence like “The man wore a hat that made him look taller,” the words “that” and “him” are both pronouns that refer to other nouns. “Him” refers to “man” that is used earlier in the sentence, but it still has meaning outside of the context in which it is used. The word “that” in the example is essentially meaningless without its context, in which it refers to the “hat” mentioned earlier to indicate that it is what makes him “look taller.”
There are some contexts, however, in which a relative pronoun can be used in an adjectival form, in which they become a relative adjective. In a sentence like “Please give me that hat,” the word “that” is now functioning as an adjective. It is still a relative pronoun in that it has no inherent meaning outside of the context, but in this setting it is functioning to modify the word “hat” in a way that specifies which one is being referred to in the sentence. Its function is much the same as an adjective like “small” or “blue” that could also describe the hat.
The role of a relative adjective is quite important, since it allows statements to have meaning without requiring repetition. Someone could say “The hat on the table is mine, please give me the hat on the table,” but the repeated phrasing sounds strange. It is more natural for someone to say “The hat on the table is mine, please give me that hat” and use a relative adjective to make the meaning clear without repetition.