In linguistics, function words (also called functors)[1] are words that have little lexical meaning or have ambiguous meaning and express grammatical relationships among 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 form important elements in the structures of sentences.
Since it was first proposed in 1952 by C. C. Fries, the distinguishing of function/structure words from content/lexical words has been highly influential in the grammar used in second-language acquisition and English-language teaching.
[3] Function words might be prepositions, pronouns, auxiliary verbs, conjunctions, grammatical articles or particles, all of which belong to the group of closed-class words.