The major word classes nouns, verbs, adjectives and adverbs are often accompanied by other elements that are dependent on them. For example, most nouns occur together with words like a, the, some, their etc., as in the following example:
Academic integrity is a term used for the professional honesty that researchers and writers of scholarly texts are expected to demonstrate in their work.
Together with such dependent elements, the major word classes form phrases. Thus, nouns and their dependents form noun phrases, verbs and their dependents form verb phrases, adjectives and their dependents form adjective phrases, and adverbs and their dependents form adverb phrases.
Phrases, in this technical sense, can be understood as expansions, or projections, of a single word. Typically, therefore, in a sentence, a phrase may be replaced by a single word, without affecting the structure of the sentence. For example:
had been teasing
Within a phrase, one word functions as the head of the phrase. The head is the only obligatory element in the phrase, and other elements in the phrase serve to enrich the meaning of the head, e.g. by adding descriptive content.
Since different types of phrases have different internal structures the type of dependents a word can combine with provides a strong clue to its word class.