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.