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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 are words that have little lexical meaning or have ambiguous meaning, but instead serve to 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.
Function words belong to the closed class of words in grammar in that it is very uncommon to have new function words created in the course of speech, whereas in the open class of words (that is, nouns, verbs, adjectives, or adverbs) new words may be added readily (such as slang words, technical terms, and adoptions and adaptations of foreign words). See neologism.
Each function word either gives some grammatical information on other words in a sentence or clause, and cannot be isolated from other words, or it may indicate the speaker's mental model as to what is being said.
Grammatical words, as a class, can have distinct phonological properties from content words. Grammatical words sometimes do not make full use of all the sounds in a language. For example, in some of the Khoisan languages, most content words begin with clicks, but very few function words do. In English, very few words other than function words begin with voiced th- [ð] (see Pronunciation of English th); English function words may have less than three letters 'I', 'an', 'in' while non-function words usually have three or or more 'eye', 'Ann', 'inn' (see three letter rule).
The following is a list of the kind of words considered to be function words:
- articles — the and a. In some inflected languages, the articles may take on the case of the declension of the following noun.
- pronouns — inflected in English, as he — him, she — her, etc.
- adpositions — uninflected in English
- conjunctions — uninflected in English
- auxiliary verbs — forming part of the conjugation (pattern of the tenses of main verbs), always inflected
- interjections — sometimes called "filled pauses", uninflected
- particles — convey the attitude of the speaker and are uninflected, as if, then, well, however, thus, etc.
- expletives — take the place of sentences, among other functions.
- pro-sentences — yes, okay, etc.
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- Fries, Charles Carpenter (1952). The Structure of English. New York: Harcourt Brace.
- Klammer , Thomas, Muriel R. Schulz and Angela Della Volpe. (2009). Analyzing English Grammar (6th ed).Longman.
- Westphal, E.O.J. (1971), "The click languages of Southern and Eastern Africa", in Sebeok, T.A., Current trends in Linguistics, Vol. 7: Linguistics in Sub-Saharan Africa, Berlin: Mouton
- Kordić, Snježana (2001). Wörter im Grenzbereich von Lexikon und Grammatik im Serbokroatischen [Serbo-Croatian Words on the Border Between Lexicon and Grammar]. Studies in Slavic Linguistics ; 18 (in German). Munich: Lincom Europa. p. 280. ISBN 3-89586-954-6. LCCN 2005530313. OCLC 47905097. OL 2863539W. Summary.