A word that’s primarily defined either as a noun or an adjective sometimes moonlights as the other part of speech. Within one sentence, each word can only function in one grammatical role at a time. With a solid understanding of what a noun is, what an adjective is, and when the two can get confused, you’ll be able to figure out what role the questionable word is playing.

Nouns

In elementary school, you may have learned that a noun is a person, place or thing. That’s still true, but you’ve got to check the word’s relationship to other words before you can be sure that it’s acting as a noun in a sentence. To be a noun, it must be the subject or object of a verb (“horse” in “The horse jumped” and “I jumped the horse”), the object of a preposition (“horse” in “I jumped over the horse”), a word that renames the subject (“mare” in “The horse is a mare” or “The horse, a mare, ate oats,” and “king” in “They made the man king”) or, finally, a direct address (“honey” in “Honey, I told you to be home by five”).

Adjectives

The elementary definition of “adjective” as “a describing word” is a good place to start. More specifically, adjectives modify nouns or other adjectives by describing their qualities or quantities, or by pinpointing how they differ from other things. In sentences, adjectives usually come before nouns or other adjectives (“red” in “The red wagon is broken,” and both “broken” and “red” in “I fixed the broken red wagon”), or they come after a linking verb (“red” in “The wagon is red”).

Nouns Used as Adjectives

English loves to make words that were originally nouns act as adjectives, modifying other nouns. For instance, the word “history” is primarily a noun (that’s its only dictionary definition). But in the term “history book,” “history” acts as an adjective for “book.” Technical writing is full of strings of nouns used adjectivally, such as the term “transportation operation instructions.” These constructions can be difficult to read quickly, especially for non-native English speakers.

Adjectives Used as Nouns

Most adjectives modify nouns or other adjectives, but a substantive adjective actually replaces a noun in a sentence. Substantive adjectives will only communicate meaningfully if listeners can understand what noun they replace. For example, in the television show title “The Young and the Restless,” the adjectives “young” and “restless” are modifying missing nouns -- so we understand the title to mean, “The People Who Are Young and the People Who Are Restless.” Because the nouns are cut out (or, as linguists say, “elided”), the adjectives are doing the nouns’ job in the sentence.