independent clause dependent clause compound clause conjunction compound clause adverbial adjunct argument predicate phrase sentence independent clause 1 compound clause 11 conjunction 1 compound clause 12 compound clause 111 conjunction 2 compound clause 112 sentence S1 for clause 111 conjunction for S1 & S2 sentence S2 for clause 112 conjunction for (S1-S2) & S3 sentence S3 for clause 12 sentence for S1 conjunctive for S1 & S2 sentence for S2 sentence for S3 realis of assertion assertion of sentence adversative of (S1-S2) & S3

Section 5-1 THE SENTENCE IN A GT GRAMMAR

With the PS-rules of a TG grammar established formally as a calculus, the linguist should be prepared to examine how they might be written to show the analyis of the sentence. The intent of this section is to justify the construction some of the specific PS-rules necessary to describe compound sentences.

PS-rule for a sentence.  
Suppose the GT linguist wants to describe a sentence. It seems that the first thing easily stated is that a pro-sentence constitutes a legitimate form for the sentence.
[P1a] Sentence   Pro-sentence
There is, of course, the possibility that the sentence consists of an independent clause.
[P1a] Sentence   Clause
Since these are mutually exclusive alternatives, we may write the two rules as a single one:
Even though the language of object and class modelling is more graphic with respect to classes and constituents by representing them as words in boxes, the student must nevertheless be careful to interpret such representations as the symbols of the grammar. Notice that the boxes colored tan are the ones representing alternative structural classes, i.e., components that contain smaller components. These are the components that relate to their superior components with the “is a” relation, yet they have in turn constituents that they relate to with the “has a” relation.

PS-rules for a clause.  
By comparing a sentence with other sentences and their parts the analyst must decide on its most appropriate phrase structure. The linguist uses the grammar to propose rules that will define the phrase structure. To see how one might proceed consider describing the following three sentences:
(1)This is John.
(2)And this is Mary.
(3)But this is Bill.
The structures of these three sentences are very similar. These sentences are independent clauses. With these three instances in mind the linguist may define a sentence in terms of a particular structure, i.e., as another clause that in some instances also has a grammatical connective (traditionally called a conjunction) such as and or but to connect it to the preceding clause or to the structure of some other proposition in the context. In this way the grammar might contain these phrase structure rules:
[P2a] Clause   X
[P2b] Clause   Conjunction + Clause
The intent is that rule [P2a] be capable of describing the structure of (1) and that rule [P2b], the structure of either (2) or (3). (We discuss the proper constituents of a clause later, but put the variable X in [P2a] to indicate that this is where they would go.)

Ellipsis as a tool of analysis.  
It may well be that the linguist will view (1) of the preceding paragraph as an ellipsis. An ellipsis results from an analysis in which it is assumed that the author has omitted something, sometimes a single word or sometimes even whole structures. Perhaps the sentence in (1) is a more explicit version of (2); after all, we feel that the normal function of a connective (conjunction) is to connect things.
(1)This is John and this is Mary.
(2)And this is Mary.
The argument runs something like this: motivated by having expressed the content of the first clause of (1) in “This is John” the author of (2) naturally omitted that clause as redundant. With independent reasons for wanting to describe (1) as well as (2), and wanting to keep the description simple, the linguist considers next how to adjust [P1b] to describe both:
[P2bi] Clause   Clause + Conjunction + Clause
[P2bii] Clause   (Clause) Conjunction + Clause
The parentheses in a phrase structure rule mean that the enclosed elements may or may not be realized as a part of the structure. When we use boxes, the optional elements are outlined with a dashed line. The question is whether the rule [P2b] should describe the connective as belonging with the second clause or not.
P2bi1: 
 
P2bii1:2:
The second rule allows the derivation of two structures for the main clause, yet the question is whether the second of these structures is really needed to describe sentences in English.

A transformational approach.  
Transformationalists are inclined to avoid this question by adopting the policy of introducing connectives by means of transformations. The idea is to use a double base transform to conjoin the clauses as we did in describing the predicate calculus in chapter 3. What this policy ignores, however, is the primacy of deep structure, i.e., the primitive sentences, and the philosophy that transformations, i.e., the rules of inference, should not introduce otherwise undefined symbols, so as to avoid the extra interpretive apparatus this would require. The alternative is to generate the category of connective in the phrase structure rule as we have in [P2b]. It is possible to leave out the parentheses. Doing so would still make the structure of “And this is Mary” possible by the use of an ellipsis transformation.

Using recursion.  
Now what about “But this is Bill”? It would appear that in the context of “This is John” and “And this is Mary,” that “But this is Bill” could be an elliptical version of sentence (1).
(1)This is John and this is Mary, but this is Bill.
The connectives keep adding clauses to the previous context. This situation seems to be further evidence that [P2] is fully adequate only by being recursive. In addition recursion would enable the description of such sentences as continue to include additional connected clauses ad infinitem.
[P2] The clause
Notice that the tan box convention requires that the designer be explicit in putting elements on the right side of [P2] together as a structural unit. The first case (tan) is for the independent clause and any further clauses compounded as parts of it. The second case (turquoise) is for the dependent clause, which has the same structure as the independent clause. The student will find the dependent clause in later rules and these are realized as 1) a noun clause [P19], 2) an adjective clause [P29], or 3) an adverbial clause [P31]. Note that every alternative structural class needs a separate box. In this system the elements that alternate cannot also be contained as ordered constituents. This is often what motivates the creation of separate rules.
Through recursion the single rule [P2] has the advantage of describing infinitely many structures as required. This rule together with [P7] allows the derivation of any of the following three structures from the main clause (Clause1):
Either clause in the first structure may likewise have one of these structures. If it has the first, the possibilities continue until the clause has either the second or the third structure. Suggestions for further phrase structure rules to define the structure of the other elements appear in subsequent chapters. The rules analyze further each of these components until the result is a so-called terminal constituent. In the grammatical calculus of sentence syntax these are the various lexical categories. In the BNF and UML diagrams the labels are in lower case Arial and the corresponding boxes are light green. The conjunction and the subject case marker are such elements. In the latter case the constituent has no separate morphological realization, but, as will become clear later, is required to reflect the syntactic environment of the argument for the use of such rules.

Syntactic structure of a compound sentence.  
In terms of [P1] the apparent phrase structure of the single sentence in “This is John and this is Mary, but this is Bill” would be the one in figure 1. The device of a triangle in tree diagrams indicates where there is structure that the linguist will have to define by creating additional rules. In box diagrams the device of colored lines serves this purpose. The dashed lines connecting to terminal constituents also indicate that lexical insertion rules are yet to be given.
Structure of compound sentence

Ellipsis vs. the null constituent.  
[P2] appears to be successful in describing the phrase structure of the sentences in (1), (2), and (3), but (4) in the context of (1) and (5) in the context of (1) and (2) seem to require more.
(1)This is John.
(2)This is John and this is Mary.
(3)This is John and this is Mary, but this is Bill.
(4)And this is Mary.
(5)But this is Bill.
There are at least three ways to go about describing ellipsis in a GT grammar. The first way (when possible) is to make the constituent optional. A second way is with a transformation deriving (4) and (5) from (2) and (3) respectively. Suppose the linguist observes a sentence beginning with a connective, in a situation where no context is reconstructable. Thirdly, one may prefer to describe such an ellipsis by positing a null constituent. This constituent is a terminal symbol (Ø) that by convention may be the realization of any constituent. Presumably any constituent has conditions where ellipsis may occur, and so there need be no explicit PS rule to introduce it. The structure of (4) and (5) would, with this method, take the form of the ones shown in figure 2.
Syntactic structure of sentences using null element
It turns out that the first way, making the constituent element(s) optional, must sometimes still make use of the null constituent. Suppose we take the first clause of a compound as an optional constituent. In this case there would seem to be no straightforward way for the dependent clause, whose connective is context sensitive, to be included in this rule. The conjunction would then have to be null whenever the first clause of a compound was omitted.
[P2'] The independent clause

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