Subject, Predicates, and The Copula
It is generally assumed that the Noun Phrase occurring with the Verb Phrase, constituting a sentence, is a subject. Copular sentences challenge this view. In a particular class of copular sentences, called "inverse copular sentences", the noun phrase which occurs with the verb phrase plays the role of predicate, occupying the position which is canonically reserved for subjects, and the subject is embedded in the verb phrase (cf. copula). This can be exemplified by pairs of sentences like these pictures of the wall are the cause of the riot (where the preverbal Noun Phrase plays the role of subject and the post-verbal one plays the role of predicate) vs the cause of the riot is these pictures of the wall (where the order is inverse). This has far reaching consequences, affecting for example the theory of expletive subjects and unaccusative verbs (cf. Moro 1997 and Hale - Keyser 2003 and references cited there).
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