Given a construction schema involving a component in a certain function which a rule of grammar requires to be occupied, and given an instantiation of this construction whose meaning provides for nothing in the respective function, then an element instantiating the component in question without corresponding to anything in the meaning of the construction is expletive (Latin, ‘filling out’). Typical expletive elements are proforms occupying the subject position of their clause.

The verb of E1a is avalent, i.e. its meaning provides for nothing in subject position. In E1b, the subordinate clause is the subject of the main clause; but as it has vacated the subject position. In both cases, the pronoun it is expletive in the subject function of these constructions.

A test for the expletive function of a proform in a construction replaces it by a proform with which it commutes in the construction schema in question. For instance, the personal pronoun it is generally in opposition or free variation with that. If the substitution fails, the proform is expletive in this construction.

Expletive elements are sometimes called dummies or placeholders and represented by △ in a structure diagram.