What is "pro-form"

n. (context grammar English) A word that substitutes for words, phrases, clauses, or sentences, whose meaning is recoverable in context.

A pro-form is a type of function word or expression that stands in for (expresses the same content as) another word, phrase, clause or sentence where the meaning is recoverable from the context. They are used either to avoid repetitive expressions or in quantification (limiting the variables of a proposition).

Pro-forms are divided into several categories, according to which part of speech they substitute:

• A pronoun substitutes a noun or a noun phrase, with or without a determiner: it, this. (Compare also prop-word; this denotes a word like one in "the blue one".)
• A pro-adjective substitutes an adjective or a phrase that functions as an adjective: so as in "It is less so than we had expected."
• A pro-adverb substitutes an adverb or a phrase that functions as an adverb: how or this way.
• A pro-verb substitutes a verb or a verb phrase: do.
• A pro-sentence substitutes an entire sentence or subsentence: Yes, or that as in "That is true".

An interrogative pro-form is a pro-form that denotes the (unknown) item in question and may itself fall into any of the above categories.

One of the most salient features of many modern Indo-European languages is that relative pro-forms and interrogative pro-forms, as well as demonstrative pro-forms in some languages, have identical forms. Consider the two different functions of who in "Who's the criminal who did this?" and "Adam is the criminal who did this".

Most other language families do not have this ambiguity and neither do several ancient Indo-European languages. For example, Latin distinguishes the relative pro-forms from the interrogative pro-forms, while Ancient Greek and Sanskrit distinguish between all three: relative, interrogative and demonstrative pro-forms.

The rules governing allowable syntactic relations between certain pro-forms (notably personal and reflexive/reciprocal pronouns) and their antecedents have been studied in what is called binding theory.