The unified theory of copular sentences

Copular sentences are sentences containing the copula. A copular sentence may contain 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....

, the copula and another 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....

. A subfield of research which has been particularly studied is the case of the copula cooccurring with two noun phrases. Along with copular sentences displaying the canonical order of predication (the subject

Subject (grammar)

The subject is one of the two main constituents of a clause, according to a tradition that can be tracked back to Aristotle and that is associated with phrase structure grammars; the other constituent is the predicate. According to another tradition, i.e...

 preceding the predicate

Predicate (grammar)

There are two competing notions of the predicate in theories of grammar. Traditional grammar tends to view a predicate as one of two main parts of a sentence, the other being the subject, which the predicate modifies. The other understanding of predicates is inspired from work in predicate calculus...

), such as A picture of the wall is the cause of the riot, there can also be "inverse copular sentences", which appear to display the order predicate-subject, such as The cause of the riot is a picture of the wall (cf. Moro 1997, Everaert et al. 2006 and references cited there). Although these two sentences are superficially very similar it can be shown that they exhibit very different properties. So, for example, it is possible to form a sentence like Which riot do you think that a picture of the wall is the cause of? but not Which wall do you think that the cause of the riot was a picture of?. The distinction between canonical and inverse copular sentences - and the unified theory of copular sentences associated with it - has been shown to be valid in various languages, and has led to some refinement of the theory of clause structure. In particular, it challenges one of the major dogmas of the theory of 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...

 structure, i.e. that the two basic constituents of a sentence - the Noun Phrase and the Verb Phrase - are associated with the logical/grammatical functions of subject

Subject (grammar)

The subject is one of the two main constituents of a clause, according to a tradition that can be tracked back to Aristotle and that is associated with phrase structure grammars; the other constituent is the predicate. According to another tradition, i.e...

 and predicate (cf. phrase structure rules and 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...

). In fact, copular sentences show that this axiom is not adequate on empirical grounds since the Noun Phrase that cooccurs with the Verb Phrase in a copular sentence can be the predicate and the subject

Subject (grammar)

The subject is one of the two main constituents of a clause, according to a tradition that can be tracked back to Aristotle and that is associated with phrase structure grammars; the other constituent is the predicate. According to another tradition, i.e...

 be contained in the Verb Phrase such as in the cause of the riot is a picture of the wall.

Inverse copular sentences and language acquisition

Interestingly, it has been suggested that inverse copular sentences appear to play a sharp role in setting the pro-drop parameter. In a Romance language like Italian, for example, in sentences of the type Noun Phrase Verb Noun Phrase, the verb generally agrees with the Noun Phrase on the left, with one exception: inverse copular sentences. One can construe minimal pairs like The cause of the riot is/*are these pictures of the wall vs. La causa della rivolta sono/*è queste foto del muro. These two sentences differ in that the copula is plural in Italian and singular in English. If one does not want to give up the idea that agreement is with the NP on the left as in all other sentences, then the only option is to assume that a null pronoun, technically called "pro", occurs between the copula and the Noun Phrase on the left. This could provide a positive piece of evidence to children learning Italian that a null pronoun exists. That a null pronoun can occur as a predicate and not only as a subject

Subject (grammar)

The subject is one of the two main constituents of a clause, according to a tradition that can be tracked back to Aristotle and that is associated with phrase structure grammars; the other constituent is the predicate. According to another tradition, i.e...

 must be in fact independently assumed in order to assign a proper structure to sentences like Sono io (lit. "is me"; meaning "It's me") which can by no means be considered a transformation of *Io sono, which has no meaning. This casts a sharp difference with respect to other sentences where the subject

Subject (grammar)

The subject is one of the two main constituents of a clause, according to a tradition that can be tracked back to Aristotle and that is associated with phrase structure grammars; the other constituent is the predicate. According to another tradition, i.e...

 appears on the right, such as Telefono io (lit. "telephone I"; meaning "I telephone"), which have been traditionally analyzed as a transformation from Io telefono involving a null subject pro on the right.

Inverse copular sentences and other constructions

The analysis involving the raising of a predicate phrase as in inverse copular sentences has been extended - mutatis mutandis - to other domains of syntax, such as noun phrases. Cases like that idiot of a doctor have been analyzed as a transformation from an underlying structure where a doctor is the subject

Subject (grammar)

The subject is one of the two main constituents of a clause, according to a tradition that can be tracked back to Aristotle and that is associated with phrase structure grammars; the other constituent is the predicate. According to another tradition, i.e...

 and idiot is the predicate. Crucially, this analysis suggests that there can be "copula" outside the verbal domain (realized by of, in the cited example) which can play the role of a hub for the predicate to be displaced (see Kayne 1994, Zamparelli 1995, Bennis et al. 1998).
The idea of a propredicate has also been extended to other types of constructions, such as for example sentences of the type it seems that John left or sentences of the type what I think is a mystery is numbers (see Moro 1997, Den Dikken 2006 and references cited there). So, for example, it seems that John left would be analyzed on a par with sentences of the type it's that John left, i.e. involving the occurrence of a pronoun it playing the role of a predicate rather than the subject

Subject (grammar)

The subject is one of the two main constituents of a clause, according to a tradition that can be tracked back to Aristotle and that is associated with phrase structure grammars; the other constituent is the predicate. According to another tradition, i.e...

as traditionally assumed.