Structural types of sentences depend upon the way the most important features of the sentence, and namely, predicativity, is reflected in their structure. Since predicativity has three features, they might be embodied either together in one component of a sentence, and such sentences are known as one-member sentences. As an example of one-member sentence we can give imperative sentences like Come here! Or Stand up! The other example is a group of nominative sentences, such as A nice winter day. No wind. In other languages the number of structures that are one-member may include also impersonal sentences, but in English impersonal sentences have a structure of the other type, of the type in which the personal element of predicativity is isolated from the other two and embodied in a special element. The other two features of predicativity (temporal and modal) are presented together in another component of the sentence. Such sentences in which the predicativity features are distributed between two elements of a sentence are named two-member sentences: It is cold. John speaks English. A dog runs.
In English and as much as we know in other languages no structure is found in which the three features of predicativity are embodied in three different components of sentences. Thus, we have no three-member sentences.
In case of two-member sentences the elements formalising the features of predicativity are known as the main members. The element which incorporates the personal feature of predicativity is known as the subject of the sentence. The element which incorporates the temporal and modal features of predicativity is known as the predicate of the sentence. Because semantically all three features of predicativity is a unity there exists specific attraction of the subject and the predicate. This attraction is known as predication. We shall discuss different variants of predication at the end of this section.
But a sentence may include either one predicativity centre or several. If a sentence has only one predicativity centre we call such sentences simple. All the sentences we gave as examples above are simple. If a sentence has more than one predicative centre we call it complex. But if we have two or more predicativity centres within one structure, there must be some relations between these elements. We know two types of relations: co-ordination and subordination. Co-ordination means that the predicative centres are not dependant on each other, but all of them depend upon the whole poly-predicative structure which imposes certain logical relationships between them. These logical relations might be conjunctive (or copulative), disjunctive, causative and adversative. Sentences having conjunctive relations between the predicative centres show them as co-existing and united: ***. Disjunctive relations show that only one of the predicative groups is actually true: ***. Causative relations indicate that the first predicative group describes the reason for the other. Adversative relations are used if in spite of the cause formulated by the first predicative group, we observe an unexpected result: ***.
The subordinate relations exist if one predicative group includes one or more other predicative groups as its elements: ***.
The predicative groups within a poly-predicative sentence in English grammar are traditionally called clauses. So the term sentence in English is employed to name a structure comprising one or more predicative groups, i.e. clauses. So, a simple sentence may be called a one-clause sentence.
Clauses usually are described as subject-predicate groups. But this definition is not absolutely correct. It is correct only if the clauses are two-member structures. But not only simple sentences but also clauses of complex sentences can be one-member, and thus, have neither predicate not subject: ***. Besides, the definition of clauses as subject-predicate groups stresses the fact that the clauses should have predication. But predication can have at least two variants. Predication may explicitly show all three features of predicativity and then it is named primary predication. If a group having predication is included into a complex structure, it may share temporal and modal features with the clause it is included in. Then these features may find no formal representation and become implied. Such groups with predication having no formal indication of tense and mood and employing non-finite verbal forms as their predicates are known as secondary predication structures: John saw Jane crossthe street.
The structural types listed above are defined for sentences used in isolation. If a sentence is used in a text it may be given in an abridged variant because the speaker or the writer may omit the elements of a sentence which are quite clear from the context. If the speaker or writer omits one or both main members of the sentence i.e. the subject and the predicate, the resulting structure is an elliptical sentence. One shouldn’t mix up elliptical sentences with one-member sentences. If we have an elliptical sentence we can restore the omitted elements without radical changes in the meaning of the sentence. But this procedure cannot be applied to a one-member sentence. For example, if we find in a text like When are you going to leave? — Tomorrow. A one-word sentence, we can restore it as I leave tomorrow. We have to classify this sentence as an elliptical one. But if try to replenish a sentence like Come here, please! with the indication of the doer of the action as You come here please! we receive either a different type of sentence (a declarative instead of an imperative one) or two one-member sentences you being an application.
But the structure of types of sentences show only one aspect of them. Sentences being elements of speech have their specific communicative functions which are discussed in the following section.