Above we defined a complex sentence as a sentence containing at least one subordinate clause. Any classification of complex sentences is therefore bound to be based on a classification of subordinate clauses. This will accordingly be our next task.
It might also prove expedient to have two different classifications independent of each other and based on different principles.
The first opposition in the sphere of principles would seem to be that between meaning, or contents, and syntactical function. But this opposition is not in itself sufficient to determine the possible variants of classification.
From the point of view of their general nominative features all the subordinate clauses can be divided into three categorial-semantic groups. The first group includes clauses that name an event as a certain fact. These pure fact-clauses can be called “substantive-nominal”. It can be checked by a substitute test: That his letters remained unanswered annoyed him very much. – That fact annoyed him very much. The woman knew only too well what was right and what was wrong. – The woman knew those matters well.
The second group of clauses also name an event-fact, but, as different from the first group, this event-fact is referred to as giving a characteristic to some substantive entity. They can be called “qualification-nominal”. It can be proved by the following replacement pattern: The man who came in the morning left a message. – That man left a message. Did you find a place where we could make a fire? – Did you find such kind of place?
The third group of clauses make their event-nomination into a dynamic relation characteristic of another event or a process or a quality of various descriptions. It can be called “adverbial” and can be tested by transformation: Describe the picture as you see it. – Describe the picture in the manner you see it. All will be well if we arrive in time. – All will be well on condition that we arrive in time.
The categorial features of clauses go together with their functional sentence-part features. Subordinate clauses are introduced by functional connective words which effects their derivation from base sentences. They are divided into two basic types; those that occupy a notional position in the derived clause and those that do not occupy such a position. The non-positional subordinators are referred to as pure conjunctions. Here belong such words as since, before, until, if, in case, because, so that, in order that, though, however, than, as if. The positional subordinators are in fact conjunctive substitutes. The main positional subordinators are the pronominal words: who, what, whose, which, that, where, when, why, as. Some of these words are double-functional. Also the zero subordinator can be named, whose polyfunctional status is similar to the status of the subordinator that.
Under the head of "function" we may bring either the position of a clause within a complex sentence, defined on the same principles as the position of a sentence part within a simple sentence, or (as is sometimes done) on the analogy between a clause and a part of speech performing the same function within a simple sentence. Besides, for certain types of clauses there may be ways of characterising them in accordance with their peculiarities, which find no parallel in other clauses. For instance, clauses introduced by a relative pronoun or relative adverb may be termed "relative clauses", which, however, is not a point of classification.
In order to obtain a clearer idea of how these various principles would work out in practice, let us take a complex sentence and define its subordinate clauses in accordance with each of these principles. Let the sentence be this: It was unreal, grotesquely unreal, that morning skies which dawned so tenderly blue could be profaned with cannon smoke that hung over the town like low thunder clouds, that warm noontides filled with the piercing sweetness of massed honeysuckle and climbing roses could be so fearful, as shells screamed into the streets, bursting like the crack of doom, throwing iron splinters hundreds of yards, blowing people and animals to bits. (M. MITCHELL) Let us first look at the two subordinate clauses introduced by the conjunction that: (1) that morning skies. .. could be profaned with cannon smoke, (2) that warm noontides.. , could be so fearful. From the point of view of meaning they may be called declarative clauses, or subordinate statements, las they contain statements which are expressed in subordinate clauses. From the point of view of function they may be termed, if we consider them as something parallel to parts of a simple sentence, either appositions to the impersonal it which opens the sentence, or subject clauses, if we take the view that the it is merely an introductory subject, or a "sham" subject, as it is sometimes called. If, last not least, we wish to compare the clauses to the part of speech which might perform the corresponding function in a simple sentence, we may call them noun clauses, or substantive clauses, which is a very usual way of treating them in English school grammars.
Now let us turn to the clause coming after the noun skies of the first subordinate clause: which dawned so tenderly blue. From the viewpoint of meaning this clause can also be said to be declarative, or a subordinate statement. It may also be termed a relative clause, because it is introduced by a relative pronoun and has a relative connection with the noun skies (or the phrase morning skies). From the functional point of view it may be called an attributive clause, and if we compare it to the part of speech which might perform the corresponding function in a simple sentence, we may call it an adjective clause, which is also common in English school grammars. The same considerations also apply to the clause that hung over the town like low thunder clouds; it is evident from the context that the word that which opens the clause is a relative pronoun (without it the clause would have no subject). Now we take the last subordinate clause: as shells screamed into the streets, bursting like the crack of doom, throwing iron splinters hundreds of yards, blowing people and animals to bits. This again would be a declarative clause or a subordinate statement, and from the viewpoint of function it may be termed an adverbial clause, as it corresponds to an adverbial modifier in a simple sentence. More exactly, it might be termed an adverbial clause of time. Now, for the last item, if we compare it to the part of speech performing the corresponding function in a simple sentence, we might term it an adverb clause, which, however, is too close to the term "adverbial clause" to be of much use in distinguishing the two notions.
To sum up these various possibilities, we have, for the first two clauses, the following terms: declarative clause, or subordinate statement; apposition clause, or subject clause; noun clause. For the second two clauses: declarative clause, or subordinate statement; attributive clause; adjective clause. For the clause coming last: declarative, or subordinate statement; adverbial clause of time; adverb clause.
The term "relative clause" may very well be applied to any clause introduced by a relative pronoun or relative adverb.O. Jespersen devotes several chapters of his book "A Modern English Grammar" to relative clauses. In accordance with his general view that elements of language may be divided into primaries, adjuncts, and subjuncts, he treats the syntactical functions of subordinate clauses as falling under these heads: "relative clauses as primaries" and "relative clause adjuncts".
From the viewpoint of function the subordinate clauses of these types are of course quite different, yet they may be all termed "relative clauses". This makes it evident that the notion "relative clause" is not a notion of syntactic function, since it cuts right across syntactical divisions.
There remains now the classification of subordinate clauses based on the similarity of their functions with those of parts of the sentence, namely the classification of clauses into subject, predicative, object, attributive, adverbial, appositional, and parenthetical clauses. In this way the general parallelism between parts of a simple sentence and subordinate clauses within a complex sentence will be kept up; however, there is no sufficient ground for believing that there will be complete parallelism in all respects and all details: on the contrary, it is most likely that differences between the two will emerge (especially in the sphere of adverbial modifiers and adverbial clauses). Subordinate clauses may well be expected to have some peculiarities distinguishing them from parts of a simple sentence.