A pro-form is a type of function word

Function word

Function words are words that have little lexical meaning or have ambiguous meaning, but instead serve to express grammatical relationships with other words within a sentence, or specify the attitude or mood of the speaker...

 or expression that stands in for (expresses the same content as) another word

Word

In language, a word is the smallest free form that may be uttered in isolation with semantic or pragmatic content . This contrasts with a morpheme, which is the smallest unit of meaning but will not necessarily stand on its own...

, phrase

Phrase

In everyday speech, a phrase may refer to any group of words. In linguistics, a phrase is a group of words which form a constituent and so function as a single unit in the syntax of a sentence. A phrase is lower on the grammatical hierarchy than a clause....

, clause

Clause

In grammar, a clause is the smallest grammatical unit that can express a complete proposition. In some languages it may be a pair or group of words that consists of a subject and a predicate, although in other languages in certain clauses the subject may not appear explicitly as a noun phrase,...

 or sentence

Sentence (linguistics)

In the field of linguistics, a sentence is an expression in natural language, and often defined to indicate a grammatical unit consisting of one or more words that generally bear minimal syntactic relation to the words that precede or follow it...

 where the meaning

Meaning (linguistics)

In linguistics, meaning is what is expressed by the writer or speaker, and what is conveyed to the reader or listener, provided that they talk about the same thing . In other words if the object and the name of the object and the concepts in their head are the same...

 is recoverable from the context. They are used either to avoid repetitive expressions or in quantification

Quantification

Quantification has several distinct senses. In mathematics and empirical science, it is the act of counting and measuring that maps human sense observations and experiences into members of some set of numbers. Quantification in this sense is fundamental to the scientific method.In logic,...

 (limiting the variables of a proposition).

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

• A pronoun
  Pronoun
  In linguistics and grammar, a pronoun is a pro-form that substitutes for a noun , such as, in English, the words it and he...
  
   substitutes a noun
  Noun
  In linguistics, a noun is a member of a large, open lexical category whose members can occur as the main word in the subject of a clause, the object of a verb, or the object of a preposition .Lexical categories are defined in terms of how their members combine with other kinds of...
  
   or a noun phrase
  Noun phrase
  In grammar, a noun phrase, nominal phrase, or nominal group is a phrase based on a noun, pronoun, or other noun-like word optionally accompanied by modifiers such as adjectives....
  
  , with or without a determiner: it, this.
• A pro-adjective substitutes an adjective
  Adjective
  In grammar, an adjective is a 'describing' word; the main syntactic role of which is to qualify a noun or noun phrase, giving more information about the object signified....
  
   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
  Adverb
  An adverb is a part of speech that modifies verbs or any part of speech other than a noun . Adverbs can modify verbs, adjectives , clauses, sentences, and other adverbs....
  
   or a phrase that functions as an adverb: how or this way.
• A pro-verb
  Pro-verb
  In grammar, a pro-verb is a word or phrase that stands in place of a verb . It does for a verb what the more widely known pronoun does for a noun. It, along with pronouns and some other word classes, form the general group of word classes pro-forms. It is a type of anaphora...
  
   substitutes a verb
  Verb
  A verb, from the Latin verbum meaning word, is a word that in syntax conveys an action , or a state of being . In the usual description of English, the basic form, with or without the particle to, is the infinitive...
  
   or a verb phrase
  Verb phrase
  In linguistics, a verb phrase or VP is a syntactic unit composed of at least one verb and the dependents of that verb. One can distinguish between two types of VPs, finite VPs and non-finite VPs . While phrase structure grammars acknowledge both, dependency grammars reject the existence of a...
  
  : do.
• A pro-sentence
  Pro-sentence
  A pro-sentence is a function word or expression that substitutes for a whole sentence whose content is recoverable from the context. Pro-sentences are a kind of pro-forms and are therefore anaphoric....
  
   substitutes an entire sentence or subsentence: Yes or (some have argued) that as in "That is true."

An interrogative pro-form

Interrogative word

In linguistics, an interrogative word is a function word used for the item interrupted in an information statement. Interrogative words are sometimes called wh-words because most of English interrogative words start with wh-...

 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

Indo-European languages

The Indo-European languages are a family of several hundred related languages and dialects, including most major current languages of Europe, the Iranian plateau, and South Asia and also historically predominant in Anatolia...

 is that relative pro-form

Relative pronoun

A relative pronoun is a pronoun that marks a relative clause within a larger sentence. It is called a relative pronoun because it relates the relative clause to the noun that it modifies. In English, the relative pronouns are: who, whom, whose, whosever, whosesoever, which, and, in some...

s 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

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 distinguishes the relative pro-forms from the interrogative pro-forms, while Ancient Greek

Ancient Greek

Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

 and Sanskrit

Sanskrit

Sanskrit , is a historical Indo-Aryan language and the primary liturgical language of Hinduism, Jainism and Buddhism.Buddhism: besides Pali, see Buddhist Hybrid Sanskrit Today, it is listed as one of the 22 scheduled languages of India and is an official language of the state of Uttarakhand...

 distinguish between all three: relative, interrogative and demonstrative pro-forms.

Table of correlatives

L. L. Zamenhof

L. L. Zamenhof

Ludwig Lazarus Zamenhof December 15, 1859 – April 14, 1917) was the inventor of Esperanto, the most successful constructed language designed for international communication.-Cultural background:...

, the inventor of Esperanto

Esperanto

is the most widely spoken constructed international auxiliary language. Its name derives from Doktoro Esperanto , the pseudonym under which L. L. Zamenhof published the first book detailing Esperanto, the Unua Libro, in 1887...

, called a table of systematic interrogative, demonstrative

Demonstrative

In linguistics, demonstratives are deictic words that indicate which entities a speaker refers to and distinguishes those entities from others...

, and quantifier pro-forms and determiners in a language a table of correlatives, after the relative and demonstrative proforms, which function together as correlative

Correlative

In grammar, correlatives are words that are separated in a sentence but function together to perform a single function.In English, examples are both—and, either—or, neither—nor, the—the , so—that , and if—then. In the Romance languages, the demonstrative pro-forms function as correlatives with the...

s. The table of correlatives for English follows.

Table of correlatives

		interrogative	demonstrative		quantifier
			proximal	distal	existential	elective	universal	negative	alternative
determiner		which what	this (sg.) these (pl.)	that (sg.) those (pl.)	some	any whichever whichsoever	every each all	no	another
pronoun	human	who whom	this (one) (sg.) these (ones) (pl.)	that (one) (sg.) those (ones) (pl.)	someone somebody	anyone anybody whoever whomever whosoever whomsoever	everyone everybody all	no one nobody	another someone else somebody else
	nonhuman	what	this (one) (sg.) these (ones) (pl.)	that (one) (sg.) those (ones) (pl.)	something	anything whatever whatsoever	everything all	nothing	something else
	out of two (dual)	which	this one (sg.) these (ones) (pl.)	that one (sg.) those (ones) (pl.)	one	either whichever whichsoever	both	neither
	out of many (plural)				some one	any whichever whichsoever	each all	none
pro-adverb	location	where	here	there	somewhere	anywhere wherever wheresoever	everywhere	nowhere	elsewhere
	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	everyway	noway noways nowise nohow (col.)	otherwise
	reason	why wherefore		therefore

Some languages may have more categories. See demonstrative

Demonstrative

In linguistics, demonstratives are deictic words that indicate which entities a speaker refers to and distinguishes those entities from others...

.

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

French language

French is a Romance language spoken as a first language in France, the Romandy region in Switzerland, Wallonia and Brussels in Belgium, Monaco, the regions of Quebec and Acadia in Canada, and by various communities elsewhere. Second-language speakers of French are distributed throughout many parts...

 (irregular) and Japanese

Japanese language

is a language spoken by over 130 million people in Japan and in Japanese emigrant communities. It is a member of the Japonic language family, which has a number of proposed relationships with other languages, none of which has gained wide acceptance among historical linguists .Japanese is an...

 (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 do not distinguish interrogative and indefinite pro-forms. In Mandarin

Standard Mandarin

Standard Chinese or Modern Standard Chinese, also known as Mandarin or Putonghua, is the official language of the People's Republic of China and Republic of China , and is one of the four official languages of Singapore....

, "Shéi yǒu wèntí?" means either "Who has a question?" or "Does anyone have a question?", depending on context.