A pro-form is a function word that substitutes a word, phrase, clause, or sentence whose meaning is recoverable from the context, and it is used to avoid redundant expressions. A pro-form is also used for the item questioned in a question, and such a pro-form is called an interrogative pro-form.

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.
• A pro-adjective substitutes an adjective or a phrase functioning as an adjective.
• A pro-adverb substitutes an adverb or a phrase functioning as an adverb.
• A pro-verb substitutes a verb or a verb phrase.
• A pro-sentence substitutes an entire sentence or subsentence.

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 man 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

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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. Note that while some categories are highly irregular, others (like quantifiers) are not.

Table of correlatives

		interrogative	demonstrative		quantifier
			proximal	distal	indefinite		universal	negative
					exclusive	inclusive
determiner		which what	this (sg.) these (pl.)	that (sg.) those (pl.)	some	any whichever whichsoever	every	no
pronoun	human	who whom	this (sg.) these (pl.)	that (sg.) those (pl.)	someone somebody	anyone anybody whoever whomever whosoever whomsoever	everyone everybody	no-one nobody
	nonhuman	what	this (sg.) these (pl.)	that (sg.) those (pl.)	something	anything whatever whatsoever	everything	nothing
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. For example, while English demonstratives only distinguish proximal (close to the speaker: this, here) and distal (far from the speaker: that, there), Japanese makes a three-way distinction between proximal (close to the speaker: kore, koko), medial (close to the addressee: sore, soko), and distal (far from both: are, asoko). Early Modern English made a similar distinction between this/here, that/there, and yon/yonder. Spanish and other Romance languages show a similar three-way distinction, which dates back to Latin.

See also

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• Deixis
• Pro-drop language

Parts of speech