A syntactic expletive (abbreviated EXPL) is a form of expletive: a word that in itself contributes nothing to the semantic meaning of a sentence, yet does perform a syntactic role. Expletive subjects in the form of dummy pronouns 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). 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 without an antecedent in Latin, Lowth declared the usage to be incorrect in English. It is possible to rephrase such sentences omitting the syntactic expletive "it," for example:
Since subject pronouns are not used in Latin except for emphasis, neither are expletive pronouns and the problem does not arise. For example, the Latin equivalent of it is necessary that you [do something], oportet tibi, translates to 'necessitates to you'.
Since English syntax and Latin syntax are not the same, the sentence was and is fully acceptable to native speakers of English and thus was and is widely considered to be proper grammar. 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, for example:
The nomenclature used for the constituents of sentences such as this is still a matter of some dispute, but there might be the subject, are the copula, and ten desks a predicate nominal.