A pro-sentence is a sentence where the subject pronoun has been dropped and therefore the sentence has a null subject.[1]
Languages differ within this parameter, some languages such as Italian and Spanish have constant pro-drop, Finnish and Hebrew for example are partial pro-drop languages and Japanese and Tamil fall into the category of discourse or radical pro-drop languages.[2] There are also languages such as English, German and Swedish that only allow pro-drop within very strict stylistic conditions.[3] A pro-sentence is a kind of pro-form and is therefore anaphoric.
In English, yes, no and okay are common pro-sentences. In response to the question "Does Mars have two moons?", the sentence "Yes" can be understood to abbreviate "Mars does have two moons."
Pro-sentences are sometimes seen as grammatical interjections, since they are capable of very limited syntactical relations. But they can also be classified as a distinct part of speech, given that (other) interjections have meanings of their own and are often described as expressions of feelings or emotions.
See main article: Yes and no. In some languages, the equivalents to yes and no may substitute not only a whole sentence, but also a part of it, either the subject and the verb, or the verb and a complement, and can also constitute a subordinate clause.
The Portuguese word sim (yes) gives a good example:
Q: Portuguese: Ela está em casa?
A: Portuguese: Acredito '''que sim'''. (literally, that yes)
Portuguese: Ela não saiu de casa, mas '''o John sim'''.
(literally, John yes).
In some languages, such as English, yes rebuts a negative question, whereas no affirms it. However, in Japanese, the equivalents of no rebut a negative question, whereas the equivalents of yes affirm it.
Q: Japanese: わかりません でした か？
A: Japanese: はい、 わかりません でした。, literally
Some languages have a specific word that rebuts a negative question. German has German: [[wikt:doch#German|doch]], French has French: [[wikt:si#French|si]], Norwegian has Norwegian: [[wikt:jo#Norwegian|jo]], Danish has Danish: [[wikt:jo#Danish|jo]], and Swedish has Swedish: [[wikt:jo#Swedish|jo]], and Hungarian has Hungarian: [[wikt:de#Hungarian|de]]. The English words "yes" and "no" were originally only used to respond to negative questions, while "yea" and "nay" were the proper responses to affirmative questions; this distinction was lost at some time in Early Modern English.
Q: German: Bist du nicht müde?
A: German: Doch. Ich gehe bald schlafen.
The prosentential theory of truth developed by Dorothy Grover,[4] Nuel Belnap, and Joseph Camp, and defended more recently by Robert Brandom, holds that sentences like "p" is true and It is true that p should not be understood as ascribing properties to the sentence "p", but as a pro-sentence whose content is the same as that of "p." Brandom calls " . . .is true" a pro-sentence-forming operator.[5]