If we are going to do more than simply accept the traditional parts of speech uncritically, we need to establish some sort of theory of word categories, a set of principles that will let us decide where the traditional categories work and where they need revision. Armed with this procedure, we will find that traditional grammars describe some categories that have no real existence in Present-day English. They also conflate other categories which are actually distinct.

Our basic procedure will be to look for elements that are grammatically distinct in English. In other words, we must find structural reasons to distinguish one item from another. For example, we can justify distinguishing verbs from nouns based on the relationships they enter into:

(7) Brown should denounce the need to memorize grammatical definitions.
(8) Brown's denunciation of the need to memorize grammatical definitions.

In example (7), denounce belongs to a category (verb) that can take an -ed inflection to indicate past time (for example, "Brown denounced it."). It can also follow an auxiliary verb (in this instance, should). It can also, in turn, be followed by a noun phrase (the need) that functions as something called the direct object. (Don't worry if some of these terms are unfamiliar. We will cover them in the upcoming chapters.) One way to speak about these possibilities is to say that denounce can enter into a variety of structural relationships with other elements in a sentence. These relationships are not a matter of the word's meaning. Notice that a wide variety of different words can replace denounce. If we were to substitute them, the sentence's meaning would change entirely. Yet all those words appear in the same structural contexts.

The word denunciation in (8) enters into an entirely different series of relationships, even though its meaning is quite similar to that of denounce. It can be preceded by a definite article (the) or a noun phrase marked with the so-called "possessive" ('s), it can take a plural -s inflection, and it can be modified by an adjective (for example, "Brown's quick denunciation). If we try to make denunciation fit into any of the patterns that work for denounce, we get ungrammatical nonsense:

(8a) *Brown's denunciationed of the need to memorize grammatical definitions.
(8b) *Brown's can denunciation of the need to memorize grammatical definitions.
(8c) *Brown's denunciation the need to memorize grammatical definitions.

As a result, we say that denunciation belongs to a different category (noun).

We will use both procedures repeatedly both to explain how we arrive at our categories and to figure out which category any particular word belongs to.

Another important point about word categories is that they exist within a hierarchy. That is, we will recognize both primary categories and subcategories. For example, we accept the primary category of noun, but not all nouns behave the same way. Words like Gina and car are both nouns and share properties such as the ability to appear as the principal word in a subject. But they also differ in the words that appear with them. Car, as long as it is singular, must appear with a word like the or a. Gina, on the other hand, cannot appear with these words:

(9) *Car is in the driveway.
(10) *The Gina was late for work.

We therefore say that car and Gina belong to the same primary category, but different subcategories.