A syntactic expletive (abbreviated EXPL) is a word that performs a syntactic role but contributes nothing to meaning. 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:
is important that you work hard for the exam."
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 said that the usage was incorrect in English. By this approach, the correct phrasing (with the omission of the syntactic expletive "it") would be:
"That you work hard for the exam is important."
Contrast it is necessary that you ... with its Latin equivalent oportet tibi, 'necessitates to you'. Since subject pronouns are not used in Latin except for emphasis, neither are expletive pronouns and the problem does not 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:
are ten desks here."
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 the word here in the example above shows the semantic emptiness of there.