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. 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. ... An interrogative word (also known simply as an interrogative) is a function word used for the item questioned in a question. ...

Pro-forms are divided into several categories according to which part of speech they substitute: In grammar, a part of speech or word class is defined as the role that a word (or sometimes a phrase) plays in a sentence. ...

• 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.

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. In linguistics and grammar, a pronoun is a pro-form that substitutes a noun or noun phrase with or without a determiner, such as you and they in English. ... A noun, or noun substantive, is a word or phrase that refers to a person, place, thing, event, substance or quality. ... In linguistics, a noun phrase is a phrase whose Head is a noun. ... Determiners are words which quantify or identify nouns. ... An adjective is a part of speech which modifies a noun, usually making its meaning more specific. ... An adverb is a part of speech that normally serves to modify verbs, adjectives, other adverbs, clauses, and sentences. ... A verb is a part of speech that usually denotes action (bring, read), occurrence (to decompose (itself), to glitter), or a state of being (exist, live, soak, stand). Depending on the language, a verb may vary in form according to many factors, possibly including its tense, aspect, mood and voice. ... A verb phrase is a phrase whose head is a verb. ... Dr. Ludovic Lazarus (Ludwik Lejzer) Zamenhof (December 15, 1859–April 14, 1917) was a Polish-Jewish ophthalmologist, philologist, and Zionist, and the initiator of Esperanto, the most widely spoken planned language to date. ... Esperanto flag Esperanto is a constructed international language. ... Demonstratives are words that indicate which objects a sentence is referring to. ... In language and logic, quantification is a construct that specifies the extent of validity of a predicate, that is the extent to which a predicate holds over a range of things. ...

Some languages may have more categories. For example, while English demonstratives only distinguish proximal (close to the speaker, e.g. this, here) and distal (far from the speaker, e.g. that, there), Japanese makes a three-way distinction between proximal (close to the speaker, e.g. kore, koko), medial (close to the addressee, e.g. sore, soko), and distal (far from both, e.g. are, asoko). Early Modern English made a similar distinction between this/here, that/there, and yon/yonder. Spanish, as well as other Romance languages, shows this same three-way distinction, dating back to Latin. Early modern English is a name for the modern English language the way it was used between around 1485 and 1650. ... The Romance languages, also called Romanic languages, are a subfamily of the Italic languages, specifically the descendants of the Vulgar Latin dialects spoken by the common people evolving in different areas after the break-up of the Roman Empire. ... Latin is the language that was originally spoken in the region around Rome called Latium. ...

One of the most salient features of 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." Proto-Indo-European Indo-European studies The Indo-European languages include some 443 (SIL estimate) languages and dialects spoken by about three billion people, including most of the major language families of Europe and western Asia, which belong to a single superfamily. ... A relative pronoun is a pronoun that introduces an adjective clause. ...

Most other language families don't 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. The Greek language (Greek Ελληνικά, IPA // – Hellenic) is an Indo-European language with a documented history of some 3,000 years. ...

See also

• Pro-drop language
• Deixis