A unified theory of copular sentences

Along with copular sentences where the canonical order of predication is displayed - that is, the subject precedes the predicate - as in a picture of the wall is the cause of the riot there can also be "inverse copular sentences" where this order is mirrored as in the cause of the riot is a picture of the wall (cf. Everaert et al 2006). Although these two sentences are superficially very similar it can be shown that they embody 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 these two types of sentences, technically referred to as "canonical" vs. inverse copular sentences, respectively - and the unified theory of copular sentences associated to it - has been proved to be valid across-languages and has lead to some refinement of the theory of clause structure. In particular it challenges one of the major dogmas of the theory of clause structure, i.e. that the two basic constituents of a sentence Noun Phrase and Verb Phrase are associated to the logical/grammatical functions of subject and predicate (cf. phrase structure rules and sentence (linguistics)). 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 be contained in the Verb Phrase. Interestingly, it has been suggested that inverse copular sentences appear to play a sharp role in setting the pro-drop parameter. In 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: the two sentences are one the gloss of the other with only one difference: the copula is singular in Italian and plural in English. If one does not want to give up the idea that agreement is on the left, then the only option is to assume that pro occurs between the copula and the Noun Phrase on the left. That pro can occur as a predicate must be in fact independently assumed to assign a proper structure to sentences like sono io (is me: "it's me") which can by no means be considered a transformation of *io sono, which has no meaning.