The word expletive is currently used in three senses: syntactic expletives, expletive attributives, and "bad language".
The word expletive comes from the Latin verb explere, meaning "to fill", via expletivus, "filling out". It was introduced into English in the seventeenth century to refer to various kinds of padding—the padding out of a book with peripheral material, the addition of syllables to a line of poetry for metrical purposes, and so forth. Use of expletive for such a meaning is now rare. Rather, expletive is a term in linguistics for a meaningless word filling a syntactic vacancy (syntactic expletives). Outside linguistics, the word is much more commonly used to refer to "bad language". Some linguists use it to refer to meaningless, "filler" use of "bad language" (expletive attributives), distinguishing this from meaningful use.
Syntactic expletives are words that perform a syntactic role but contribute nothing to meaning. Expletive subjects are part of the grammar of many non-pro-drop languages such as English, whose clauses normally require overt provision of subject even when the subject can be pragmatically inferred (for an alternative theory considering expletives like there as a dummy predicate rather than a dummy subject based on the analysis of the copula see Moro 1997 in the list of references cited here). Consider this example:
Following the eighteenth-century conception of pronoun, Bishop Robert Lowth objected that since it is a pronoun, it should have an antecedent. Since it cannot function like that in Latin, Lowth said that the usage was incorrect in English. Contrast it is necessary that you ... with its Latin equivalent oportet tibi, meaning more or less 'necessitates for you'. Since subject pronouns aren't used in Latin except for emphasis, neither are expletive pronouns and the problem doesn't arise.
Whether or not it is a pronoun here (and linguists today would say that it is one), English is not Latin; and the sentence was and is fully acceptable to native speakers of English and thus was and is grammatical. It has no meaning here; it merely serves as a dummy subject. (It is sometimes called preparatory it or prep it, or a dummy pronoun.)
Bishop Lowth did not condemn sentences that use there as an expletive, even though it is one in many sentences, for example:
The nomenclature used for the constituents of sentences such as this is still a matter of some dispute, but there might be called subject, are copula, and ten desks predicate nominal. Meanwhile here is an adverbial phrase that conveniently reveals the semantic vacuity of there in this example.
There is some disagreement over whether the it in such sentences as
is an expletive. Whereas it makes no sense to ask what the it means in "It is important that you work hard for the exam", some people might say that the dummy it in "It is raining now" means the weather (even if the word weather has not previously been mentioned). Thus the it in such sentences is sometimes called expletive, sometimes a weather "it". Compare with weather verb.
Full article ▸