Function words (or grammatical words or synsemantic words or structure-class 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.  Consider the following two sentences:
- The winfy prunkilmonger from the glidgement mominkled and brangified all his levensers vederously.
- Glop angry investigator larm blonk government harassed gerfritz infuriated sutbor pumrog listeners thoroughly.
In sentence (1) above, the content words have been changed into nonsense syllables but it is not difficult for one to posit that winfy is an adjective, prunkilmonger, glidgement, levensers as nouns, mominkled, brangified as verbs and vederously as an adverb based on clues like the derivational and inflectional morphemes. (The clue is in the suffixes: -y indicates adjectives such as "wintery"; -er, -ment and -ers indicates nouns such as "baker", "battlement" and "messengers"; -led and -fied suggests verbs such as "mingled" and "clarified"; and -ly is that of adverbs such as "vigorously"). Hence, even without lexical meaning, the sentence can be said to be rather "meaningful". However, when the reverse is done and the function words are being changed to nonsense syllables as in sentence (2), the result is a totally incomprehensible sentence as the grammatical meaning which is signaled by the structure words is not present. Hence, function words provide the grammatical relationships between the open class words and helps create meaning in 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).
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.
- 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