These are the grammars in which the sentence can be represented structurally into two stages. Obtaining different structures from sentences having the same meaning is undesirable in language understanding systems. Sentences with the same meaning should always correspond to the same internal knowledge structures. In one stage the basic structure of the sentence is analyzed to determine the grammatical constituent parts and in the second stage just the vice versa of the first one. This reveals the surface structure of the sentence, the way the sentence is used in speech or in writing. Alternatively, we can also say that application of the transformation rules can produce a change from passive voice to active voice and vice versa. Let us see the structure of a sentence as given below.
1. Ram is eating an apple (In Active Voice)
2. An apple is being eaten by Ram (In Passive Voice)
Both of the above sentences are two different sentences but they have same meaning. Thus it is an example of a transformational grammar. These grammars were never widely used in computational models of natural language. The applications of this grammar are changing of voice (Active to Passive and Passive to Active) change a question to declarative form etc.
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