In linguistics, a transformational grammar or transformational-generative grammar (TGG) is a generative grammar, especially of a natural language, that has been developed in the Chomskyan tradition of phrase structure grammars (as opposed to dependency grammars). Additionally, transformational grammar is the tradition that gives rise to specific transformational grammars. Much current research in transformational grammar is inspired by Chomsky's Minimalist Program.
Deep structure and surface structure 
In 1957, Noam Chomsky published Syntactic Structures, in which he developed the idea that each sentence in a language has two levels of representation — a deep structure and a surface structure. The deep structure represented the core semantic relations of a sentence, and was mapped on to the surface structure (which followed the phonological form of the sentence very closely) via transformations. Chomsky believed there are considerable similarities between languages' deep structures, and that these structures reveal properties, common to all languages that surface structures conceal. However, this may not have been the central motivation for introducing deep structure. Transformations had been proposed prior to the development of deep structure as a means of increasing the mathematical and descriptive power of context-free grammars. Similarly, deep structure was devised largely for technical reasons relating to early semantic theory. Chomsky emphasizes the importance of modern formal mathematical devices in the development of grammatical theory:
But the fundamental reason for [the] inadequacy of traditional grammars is a more technical one. Although it was well understood that linguistic processes are in some sense "creative," the technical devices for expressing a system of recursive processes were simply not available until much more recently. In fact, a real understanding of how a language can (in Humboldt's words) "make infinite use of finite means" has developed only within the last thirty years, in the course of studies in the foundations of mathematics.—Aspects of the Theory of Syntax
Formal definition 
Chomsky's advisor, Zellig Harris, took transformations to be relations between sentences such as "I finally met this talkshow host you always detested" and simpler (kernel) sentences "I finally met this talkshow host" and "You always detested this talkshow host". Chomsky developed a formal theory of grammar where transformations manipulated not just the surface strings, but the parse tree associated to them, making transformational grammar a system of tree automata. This definition proved adequate for subsequent versions including the `extended', `revised extended', and `Government-Binding' (GB) versions of generative grammar, but may no longer be sufficient for the current minimalist grammar in that merge may require a formal definition that goes beyond the tree manipulation characteristic of Move α.
Development of basic concepts 
Though transformations continue to be important in Chomsky's current theories, he has now abandoned the original notion of Deep Structure and Surface Structure. Initially, two additional levels of representation were introduced (LF — Logical Form, and PF — Phonetic Form), and then in the 1990s Chomsky sketched out a new program of research known as Minimalism, in which Deep Structure and Surface Structure no longer featured and PF and LF remained as the only levels of representation.
To complicate the understanding of the development of Noam Chomsky's theories, the precise meanings of Deep Structure and Surface Structure have changed over time — by the 1970s, the two were normally referred to simply as D-Structure and S-Structure by Chomskyan linguists. In particular, the idea that the meaning of a sentence was determined by its Deep Structure (taken to its logical conclusions by the generative semanticists during the same period) was dropped for good by Chomskyan linguists when LF took over this role (previously, Chomsky and Ray Jackendoff had begun to argue that meaning was determined by both Deep and Surface Structure).
Innate linguistic knowledge 
Terms such as "transformation" can give the impression that theories of transformational generative grammar are intended as a model for the processes through which the human mind constructs and understands sentences. Chomsky is clear that this is not in fact the case: a generative grammar models only the knowledge that underlies the human ability to speak and understand. One of the most important of Chomsky's ideas is that most of this knowledge is innate, with the result that a baby can have a large body of prior knowledge about the structure of language in general, and need only actually learn the idiosyncratic features of the language(s) it is exposed to. Chomsky was not the first person to suggest that all languages had certain fundamental things in common (he quotes philosophers writing several centuries ago who had the same basic idea), but he helped to make the innateness theory respectable after a period dominated by more behaviorist attitudes towards language. Perhaps more significantly, he made concrete and technically sophisticated proposals about the structure of language, and made important proposals regarding how the success of grammatical theories should be evaluated.
Grammatical theories 
In the 1960s, Chomsky introduced two central ideas relevant to the construction and evaluation of grammatical theories. The first was the distinction between competence and performance. Chomsky noted the obvious fact that people, when speaking in the real world, often make linguistic errors (e.g., starting a sentence and then abandoning it midway through). He argued that these errors in linguistic performance were irrelevant to the study of linguistic competence (the knowledge that allows people to construct and understand grammatical sentences). Consequently, the linguist can study an idealised version of language, greatly simplifying linguistic analysis (see the "Grammaticality" section below). The second idea related directly to the evaluation of theories of grammar. Chomsky distinguished between grammars that achieve descriptive adequacy and those that go further and achieved explanatory adequacy. A descriptively adequate grammar for a particular language defines the (infinite) set of grammatical sentences in that language; that is, it describes the language in its entirety. A grammar that achieves explanatory adequacy has the additional property that it gives an insight into the underlying linguistic structures in the human mind; that is, it does not merely describe the grammar of a language, but makes predictions about how linguistic knowledge is mentally represented. For Chomsky, the nature of such mental representations is largely innate, so if a grammatical theory has explanatory adequacy it must be able to explain the various grammatical nuances of the languages of the world as relatively minor variations in the universal pattern of human language. Chomsky argued that, even though linguists were still a long way from constructing descriptively adequate grammars, progress in terms of descriptive adequacy will only come if linguists hold explanatory adequacy as their goal. In other words, real insight into the structure of individual languages can only be gained through comparative study of a wide range of languages, on the assumption that they are all cut from the same cloth.
"I-Language" and "E-Language" 
In 1986, Chomsky proposed a distinction between I-Language and E-Language, similar but not identical to the competence/performance distinction. (I-language) refers to Internal language and is contrasted with External Language (or E-language). I-Language is taken to be the object of study in linguistic theory; it is the mentally represented linguistic knowledge that a native speaker of a language has, and is therefore a mental object — from this perspective, most of theoretical linguistics is a branch of psychology. E-Language encompasses all other notions of what a language is, for example that it is a body of knowledge or behavioural habits shared by a community. Thus, E-Language is not itself a coherent concept, and Chomsky argues that such notions of language are not useful in the study of innate linguistic knowledge, i.e., competence, even though they may seem sensible and intuitive, and useful in other areas of study. Competence, he argues, can only be studied if languages are treated as mental objects.
Chomsky argued that the notions "grammatical" and "ungrammatical" could be defined in a meaningful and useful way. In contrast, an extreme behaviorist linguist would argue that language can only be studied through recordings or transcriptions of actual speech, the role of the linguist being to look for patterns in such observed speech, but not to hypothesize about why such patterns might occur, nor to label particular utterances as either "grammatical" or "ungrammatical." Although few linguists in the 1950s actually took such an extreme position, Chomsky was at an opposite extreme, defining grammaticality in an unusually mentalistic way (for the time). He argued that the intuition of a native speaker is enough to define the grammaticalness of a sentence; that is, if a particular string of English words elicits a double take, or feeling of wrongness in a native English speaker, and when various extraneous factors affecting intuitions are controlled for, it can be said that the string of words is ungrammatical. This, according to Chomsky, is entirely distinct from the question of whether a sentence is meaningful, or can be understood. It is possible for a sentence to be both grammatical and meaningless, as in Chomsky's famous example "colorless green ideas sleep furiously." But such sentences manifest a linguistic problem distinct from that posed by meaningful but ungrammatical (non)-sentences such as "man the bit sandwich the," the meaning of which is fairly clear, but no native speaker would accept as well formed.
The use of such intuitive judgments permitted generative syntacticians to base their research on a methodology in which studying language through a corpus of observed speech became downplayed, since the grammatical properties of constructed sentences were considered to be appropriate data to build a grammatical model on.
From the mid-1990s onwards, much research in transformational grammar has been inspired by Chomsky's Minimalist Program. The "Minimalist Program" aims at the further development of ideas involving economy of derivation and economy of representation, which had started to become significant in the early 1990s, but were still rather peripheral aspects of Transformational-generative grammar theory.
- Economy of derivation is a principle stating that movements (i.e., transformations) only occur in order to match interpretable features with uninterpretable features. An example of an interpretable feature is the plural inflection on regular English nouns, e.g., dogs. The word dogs can only be used to refer to several dogs, not a single dog, and so this inflection contributes to meaning, making it interpretable. English verbs are inflected according to the number of their subject (e.g., "Dogs bite" vs "A dog bites"), but in most sentences this inflection just duplicates the information about number that the subject noun already has, and it is therefore uninterpretable.
- Economy of representation is the principle that grammatical structures must exist for a purpose, i.e., the structure of a sentence should be no larger or more complex than required to satisfy constraints on grammaticality.
Both notions, as described here, are somewhat vague, and indeed the precise formulation of these principles is controversial. An additional aspect of minimalist thought is the idea that the derivation of syntactic structures should be uniform; that is, rules should not be stipulated as applying at arbitrary points in a derivation, but instead apply throughout derivations. Minimalist approaches to phrase structure have resulted in "Bare Phrase Structure," an attempt to eliminate X-bar theory. In 1998, Chomsky suggested that derivations proceed in phases. The distinction of Deep Structure vs. Surface Structure is not present in Minimalist theories of syntax, and the most recent phase-based theories also eliminate LF and PF as unitary levels of representation.
Mathematical representation 
Returning to the more general mathematical notion of a grammar, an important feature of all transformational grammars is that they are more powerful than context-free grammars. This idea was formalized by Chomsky in the Chomsky hierarchy. Chomsky argued that it is impossible to describe the structure of natural languages using context-free grammars. His general position regarding the non-context-freeness of natural language has held up since then, although his specific examples regarding the inadequacy of CFGs in terms of their weak generative capacity were later disproven.
The usual usage of the term 'transformation' in linguistics refers to a rule that takes an input typically called the Deep Structure (in the Standard Theory) or D-structure (in the extended standard theory or government and binding theory) and changes it in some restricted way to result in a Surface Structure (or S-structure). In TGG, Deep structures were generated by a set of phrase structure rules.
For example, a typical transformation in TG is the operation of subject-auxiliary inversion (SAI). This rule takes as its input a declarative sentence with an auxiliary: "John has eaten all the heirloom tomatoes." and transforms it into "Has John eaten all the heirloom tomatoes?" In their original formulation (Chomsky 1957), these rules were stated as rules that held over strings of either terminals or constituent symbols or both.
- X NP AUX Y X AUX NP Y
(where NP = Noun Phrase and AUX = Auxiliary)
In the 1970s, by the time of the Extended Standard Theory, following the work of Joseph Emonds on structure preservation, transformations came to be viewed as holding over trees. By the end of government and binding theory in the late 1980s, transformations are no longer structure changing operations at all; instead they add information to already existing trees by copying constituents.
The earliest conceptions of transformations were that they were construction-specific devices. For example, there was a transformation that turned active sentences into passive ones. A different transformation raised embedded subjects into main clause subject position in sentences such as "John seems to have gone"; and yet a third reordered arguments in the dative alternation. With the shift from rules to principles and constraints that was found in the 1970s, these construction-specific transformations morphed into general rules (all the examples just mentioned being instances of NP movement), which eventually changed into the single general rule of move alpha or Move.
Transformations actually come of two types: (i) the post-Deep structure kind mentioned above, which are string or structure changing, and (ii) Generalized Transformations (GTs). Generalized transformations were originally proposed in the earliest forms of generative grammar (e.g., Chomsky 1957). They take small structures, either atomic or generated by other rules, and combine them. For example, the generalized transformation of embedding would take the kernel "Dave said X" and the kernel "Dan likes smoking" and combine them into "Dave said Dan likes smoking." GTs are thus structure building rather than structure changing. In the Extended Standard Theory and government and binding theory, GTs were abandoned in favor of recursive phrase structure rules. However, they are still present in tree-adjoining grammar as the Substitution and Adjunction operations, and they have recently re-emerged in mainstream generative grammar in Minimalism, as the operations Merge and Move.
In generative phonology, another form of transformation is the phonological rule, which describes a mapping between an underlying representation (the phoneme) and the surface form that is articulated during natural speech.
See also 
- Generalised phrase structure grammar
- Generative semantics
- Head-driven phrase structure grammar
- Heavy NP shift
- Jerzy Kuryłowicz
- Lexical functional grammar
- Parasitic gap
- Transformational syntax
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