We now have a number of ways of describing language available to us. Therefore important to understand the main features of each of these descriptions in order to consider how they can be used most appropriately in ESP courses. We can identify six main stages of development.
a.            Classical or Traditional Grammar
         Descriptions of English and other languages were based on the grammars of the classical languages, Greek and Latin. These descriptions were based on an analysis of the role played by each word in the sentence. Languages were describe in this way because the classical language were case based language where the grammatical function of each word in the sentence was made apparent by the use of appropriate inflections. The prestige of the old classical languages ensured the survival of this form of description even after English had lost most of its case markers and become a largely word order based language. It can also be argued that although cases may no longer be apparent in modern English, the concepts they represent underlie any language (Fillmore,1968). Thus a knowledge of the classical description can still deepen on our knowledge of how languages operate.
 b.        Structural Linguistics
            The structural or “Slot” and “filler” form Language description will be familiar to most Language teachers as result of the enormous influence it has had on L teaching since the Second World War. In a structural description the grammar of the language is described in terms of syntagmatic structures which carry the fundamental propositions (statement, interrogative, negative, imperative, etc) and notions (time, number, gender etc). by varying the words within these structural frameworks, sentence with different meanings can be generated. This method of linguistic analysis led in English language teaching to the development of the substitution table as a typical means of explain in grammatical patterns. The very simplicity of the structural language description entails that there are large areas of language use that it cannot explain. In particular it may fail to provide the learner with an understanding of the communicative use of the structures. Later developments in language teaching and linguistics have attempted to remedy this weakness.

There are still widely used today as this example from the Nucleus series:
Diabetes mellitus
Inadequate heat loss
Some foods
A dog bite
An electric shock
Insufficient calcium
Severe shock
Burns
A haemorrhage




Can
May

Cause


Result in


Lead to
Unconsciousness
Shock
Bad teeth
Death
Heat stroke
Blisters
Allergies
Asphyxia
Rabies

 This table explains the grammatical patterns which can be made by combining words in that table. In this table we can make sentences, the first sentence is “a dog bite can cause bad teeth.”  It is the correct grammatical but in meaning is still not correct. The seconds is “a dog bite can cause rabies,” this sentence also correct in grammatical and correct in meaning.
c.         Transformational Generative (TG) Grammar
            For ESP the most important lesson to be drawn from Chomsky’s work was the distinction he made between performance (i.e surface structures) and competence ( i.e the deep level rules). Chomsky’s own definition of performance and competence was narrowly based, being concerned only with syntax. A simple way of seeing the distinction between performance and competence is in our capacity to understand the meanings of words we have never met before. In the early stages of its Development, ESP put most emphasis on describing the performance needed for communication in the target situation and paid little attention to the Competence underlying it
d.         Language Variation and Register Analysis
The language varies based on context it is used. Sometimes it can be formal or informal and written or spoken. The text bellow shows two different styles with same purpose. The first text is a transcript of a demonstration in using the lathe while the second text is the instructions from the manual.


Text 1
Now I have to change the final size drill I require, which is three quarters of an inch diameter and this is called a morse-taper sleeve.
A slower speed for a larger drill.
Nice even feed should give a reasonable finish to the hole.
Applying the coolant periodically. This is mainly for lubrication rather than cooling.
Almost to depth now.
Right. Withdrawing the drill.
That’s fine.

Text 2
1.      Select required drill.
2.      Mount drill in tailstock. Use tapper sleeves as necessary.
3.      Set speed and start machine spindle.
4.      Position tailstock to work piece.
5.      Apply firm even pressure to tailstock hand wheel to feed drill into work piece
6.      Apply coolant frequently.
7.      Drill hole to depth.
8.      Withdraw drill.
9.      Stop machine.
Even though those texts are different in styles but both convey the same message and purpose. The first text seems less formal and contains the expressions, feelings, and comments from the doer and also it is spoken which indicates the situation of the doer in doing the demonstration while the second one is more formal and it is written.  If language varies according to context, it should be possible to identify the kind of language associated with a specific context, such as an area of knowledge (legal English; social English; medical English; business English; scientific English etc), or an area of use (technical manuals, academic texts, business meetings, advertisements, doctor-patient communication etc) which means register analysis. This will affect ESP.
e.         Functional/Notional Grammar
Functional deals with social behavior and represent the intention of the speaker/ writer like advising, warning, threatening, describing, etc  while notional deals with the reflection of the way of human mind thinks (the mind and thereby language divide reality) like time, number, gender, location, etc. It appears in 1970s when the equivalence for learning various languages in formal grounds was difficult to do because the formal structures of language shows considerable variation. These variations make a difficulty in dividing up the learning task into units of equivalent value across the various languages on the basis of formal grammar (structural). But in notional and functional grounds, some approximate equivalence can be achieved since both represent the categories of human thinking and social behavior, which do not vary across language. So in 1970s there was a move from structural ground to functional and notional grounds which means there is a move from basic grammar study in school to how to use language that students already have.  This influences the development of ESP. 
f.          Discourse/ Rhetorical Analysis
This stage deals with how the meaning between sentences is generated. The important thing is the context of the language.
I think I will go out for a walk.
It is raining
It is raining
I think I will go out for walk.
Both of the texts have context. The first sentences show that the second speaker remind the speaker not to go outside because of rain while in the second sentences, the second speaker wants to go out because of the rain. Different replacement makes different meaning. Because of this discourse analysis, the effects to ESP are:
1.            Learners are made aware of the stages in certain set-piece transaction associated with particular specialist fields. The aim of such an approach is to make the learners into more efficient readers, by making them aware of the underlying structure of a text and the way in which language has been organized to create this structure
2.            The second use of discourse analysis in ESP has been through materials which aim to explain how meaning is created by the relative positions of the sentences in written text. This has become the central feature of a large number of ESP textbook aimed at developing knowledge of how sentences are combined in text in order to produce a particular meaning.