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:
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.
Table of correlatives
L. L. Zamenhof, the inventor of Esperanto, called a table of systematic interrogative, demonstrative, and quantifier pro-forms and determiners in a language a table of correlatives, after the relative and demonstrative proforms, which function together as correlatives. The table of correlatives for English follows.
Some languages may have more categories. See demonstrative.
Note that some categories are regular and some are not. They may be regular or irregular also depending on languages. The following chart shows comparison between English, French (irregular) and Japanese (regular):
(Note that "daremo", "nanimo" and "dokomo" are universal quantifiers with positive verbs.)
Some languages do not distinguish interrogative and indefinite pro-forms. In Mandarin, "Shéi yǒu wèntí?" means either "Who has a question?" or "Does anyone have a question?", depending on context.