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 whose 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.
• A pro-adjective substitutes an adjective or a phrase functioning as an adjective: so as in "It is less so than we had expected."
• A pro-adverb substitutes an adverb or a phrase functioning 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 (some have argued) that as in "That is true."

An interrogative pro-form is a pro-form denoting the (unknown) item questioned in a 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?" or the meanings of that in "That's the movie that you saw back home."

Most other language families do not have this ambiguity, nor do several ancient Indo-European languages. For example, both Latin and Ancient Greek distinguish the relative pro-forms from the interrogative pro-forms.

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. The table of correlatives for English follows.

Table of correlatives

		interrogative	demonstrative		quantifier
			proximal	distal	existential	free choice	universal	negative
determiner		which what	this (sg.) these (pl.)	that (sg.) those (pl.)	some	any whichever whichsoever	every each all	no
pronoun	human	who whom	this (sg.) these (pl.)	that (sg.) those (pl.)	someone somebody	anyone anybody whoever whomever whosoever whomsoever	everyone everybody all	no one nobody
	nonhuman	what	this (sg.) these (pl.)	that (sg.) those (pl.)	something	anything whatever whatsoever	everything all	nothing
	out of two	which	this (sg.) these (pl.)	that (sg.) those (pl.)	one	either whichever whichsoever	both	neither
	out of many				some one	any whichever whichsoever	each all	none
pro-adverb	location	where	here	there	somewhere	anywhere wherever wheresoever	everywhere	nowhere
	source	whence wherefrom	hence	thence thencefrom		whenceever whencesoever		nowhence
	goal	whither whereto whereinto whereunto	hither	thither	somewhither	anywhither whithersoever		nowhither
	time	when	now	then	sometime	anytime whenever whensoever	always everywhen	never
	manner	how whereby	thus hereby	thereby	somehow	anyhow however howsoever		no wise nohow (col.)
	reason	why wherefore		therefore

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):

	interrogative	quantifier
		existential	negative
human	who qui dare	someone quelqu'un dareka	no one personne daremo
nonhuman	what que nani	something quelque chose nanika	nothing rien nanimo
location	where où doko	somewhere quelque part dokoka	nowhere nulle part dokomo

(Note that "daremo", "nanimo" and "dokomo" are universal quantifiers with positive verbs. )

Some languages don't 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.

See also

• Anaphora (linguistics)
• Deixis
• Pro-drop language