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A function word is a word with no lexical meaning or semantic content on its own that primarily adds grammatical information. Also known as a functor, form word, or structure-class word, this kind of word reveals the structural relationships between words in a sentence. Prepositions, conjunctions, and auxiliary verbs can all be function words and are considered important linguistic building blocks. Unlike function words, content words have specific lexical meanings.
In English, among other languages, a function word carries little meaning and is usually defined by its grammatical relationship to another word. Nouns like “dog," adjectives like “green,” and verbs like “to run” generally provide the majority of a sentence’s meaning. They are part of the open class of words because languages can easily add or remove words from this group. For example, many newer English vocabulary words like “fax,” “website,” and “email” are part of the open class.
Conversely, function words are part of the closed class because languages do not typically add anything new to this group or borrow these words from other languages. Auxiliary words like “might” and “have,” conjunctions like “whether” and “that,” and some adverbs, including “too” and “very,” are all function words in English. A function word adds grammatical information rather than meaning to a sentence. For example, a function word like “the” when paired with the noun “dog” to form "the dog" can add grammatical information but does not change the noun’s meaning.
Traditionally, open class words were assumed to determine sentence structure while a function word was seen as just an addition. For example, a phrase like “The bear will see the honey,” was broken into a noun phrase (“the bear”) and a verb phrase (“will see the honey”). The function word “the” was considered just an addition to the noun phrase.
This traditional framework shifted by the mid-1980s when function words began to be understood as the determinants of categorical status. Thus the noun phrase “the bear” was interpreted as a determiner phrase (“the”) that contained a noun phrase (“bear”). Determiners like the function word “the” became heads of determiner phrases rather than just part of a noun phrase.
Some languages, such as Mandarin, are characterized by a profusion of function words. Function words are usually not stressed and therefore speakers often contract them in cases like using “I’ve” for “I have.” Children have a tendency to drop function words from their speech.
Content words include nouns, verbs, and adjectives and have an explainable lexical meaning. Unlike function words, content words are described in terms of their specific meanings rather than their syntactic or grammatical functions. Function and content words should be seen as forming a continuum rather than two different categories because some words, like the English preposition “behind,” share characteristics of both.