Function words

The distinction between function/structure words and content/lexical words proposed by C. C. Fries in 1952 has been highly influential in the grammar used in second language acquisition and English language teaching.Function words (also called functors) are words that have little lexical meaning or have ambiguous meaning, and they express grammatical relationships with other words within a sentence, or specify the attitude or mood of the speaker. They signal the structural relationships that words have to one another and are the glue that holds sentences together. Thus, they serve as important elements to the structures of sentences.

Words that are not function words are called content words (or open class words or lexical words or autosemantic words): these include nouns, verbs, adjectives, and most adverbs, although some adverbs are function words (e.g., then and why). Dictionaries define the specific meanings of content words, but can only describe the general usages of function words. By contrast, grammars describe the use of function words in detail, but treat lexical words in general terms only.

Function words might be prepositions, pronouns, auxiliary verbs, conjunctions, grammatical articles or particles, all of which belong to the group of closed-class words. Interjections are sometimes considered function words but they belong to the group of open-class words. Function words might or might not be inflected or might have affixes.

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