Today it is a challenge to get a proper solution in terms of linguistics. Significant contribution to the development of this problem has made A. A. Shakhmatov

1.2 Types of sentences
The idea of ​​the essence of predicativity (like the term itself) is not unambiguous. The term "predicativity" also means the property of a predicate as a syntactic member of a two-part sentence. The concept of predicativity is included in the notions of "predicative connection", "predicative relations", which denote the relations connecting the subject and predicate, as well as the logical subject and predicate; In this usage predication is interpreted no longer as a category of the highest stage of abstraction (inherent in the model of the sentence as such, in general, regardless of its composition), but as a concept related to the level of sentence division, that is, with such sentences in which the subject and the predicate.[5, 58]
Thus, it is important to distinguish between these concepts of predicativity. When qualifying the grammatical meaning of a simple sentence, the term "predicativity" is understood as a syntactic category.
A simple sentence is part of a paradigmatic and syntagmatic relationship. Paradigmatic relations are formal changes in the design itself (particular manifestations of the general categorical meaning), expressed by special means. Syntagmatic relations are a combination of members of a simple sentence with each other according to certain rules.
The subject and predicate, with the minor members attached to them, form two constituent parts or two "composition" of the sentence: the group ("composition") of the subject and the group ("composition") of the predicate. On the basis of their representation in the external structure of the sentences, all sentences are divided into one-part and two-part. There are also elliptical sentences.
Ellipse - This is the process of excluding the obligatory structural element of the sentences, i.e. subject or predicate, or both. The ellipsis of the subject occurs when the situation and the context of communication allow one to unambiguously identify it. As a rule, the subject is omitted in the form of the pronoun of the first person: Can't say how glad I am! In Russian, these constructions are not colloquial and are a grammatical norm, supported by a developed system of inflectional morphology, which legitimizes the absence of a subject. Sometimes in colloquial speech, the ellipsis of two main terms is allowed, while this segment of the conversation remains informational enough: When? At 5 o' clock; Where to? - To the restaurant?
There is another point of view that the elliptical sentence is created not only when the subject or predicate is omitted, but also in the absence of any secondary member of the sentence.
The point of view of A.M. Peshkovskiy is shared by LS. Barkhudarov and A.M. Mukhin. So, L.S. Barkhudarov believes that any sentence "in which at least one word is represented by the zero variant" should be considered as elliptical. For example: The two men's eyes met. In Dorians's there was pity.
"In contrast to the phrase, performing, as noted earlier, the name (nominative) function, the sentence expresses judgment or motivation for action, or a question. Grammatical basis of the sentence is, in contrast to the word combination, predicativity, i.e. expression through various means of a language of the relation of the content of the utterance to reality. By the sentences can consist of either one or several significant words"[2, 115]
The material for the study is a simple sentence of the modern English literary language in its written form. In any case, it can hardly be denied that the written form of language, in view of its important role in the life of modern society, is at least as worthy an object of scientific research as the oral form. Oral (conversational) style of speech was the object of research only insofar as it is reflected in contemporary fiction.
About components of a complex sentence, if a complex sentence is a sentence, then can it be assumed that parts of it are also sentences?
The following points of view on this question are possible. One is that "in the composition of any type of complex sentence, a simple sentence that enters into this complex whole on the rights of its part does not lose essential features of the sentences and cannot lose them, since this would turn it into some other unit, different from the sentences"[7, 48]
All the above-mentioned groups of unions appear within the compound sentence, expressing certain relations between the sentences to be joined. "But, despite the fact that we only single out five groups of creative unions, we should not think about it," she wrote. I.P. Konkova, - that the circle of semantic relations is limited only by the enumerated relations: connections, oppositions, separations, etc. Semantic nuances, expressed both with the help of unions, and with the help of semantic-syntactic means, represent a great variety for each type of communication"[26, 140]
The needs of thinking and communication created in the language special syntactic formations, consisting of the union of several sentences. These syntactic formations are called complex sentences. To understand the concept of "complex sentence", it is necessary to take into account that the structure of a complex whole is not a mechanical sum of the components of this whole of simple units. Internal connections of elements of a complex sentence are very diverse and do not fit into the scheme: "The complex consists of simple".
In the history of syntax development, the term "complex sentence" was understood differently or replaced by another. So, A.M. Peshkovskiy rejected the term "complex sentence". Instead of the term "compound sentence", the linguist uses the term "complex whole", by which he understands "Such phrases that consist of two or more sentences"[39, 123-125]
A.A. Shakhmatov uses the term "complex sentence" in a special sense: "Complex, as opposed to simple sentences, we call those sentences in which there is either a double predicate, or a secondary, or an additional predicate, or an additional secondary predicate"[49, 49]
According to A.N. Gvozdev "A complex proposition is the combination of several sentences with the help of those or other syntactic means, in a grammatical whole, serving as an expression of a finished thought. Like any sentences, a complex sentence is generally characterized by the intonation of the end. The individual sentences which lays in complex sentences do not have independence and legitimacy and are interrelated and complementary elements of a whole complex sentences"[20, 172]
MN Peterson to refer to the concept of "complex sentence" uses the term "compound phrases"[37, 28-34]
Intensive study of a complex sentence began in the second half of the 20th century, primarily in the works of V.V. Vinogradova, N.S. Pospelov, then in the works of L.Y. Maksimova, V.A. Beloshapkova , M.I. Cheremisina and other researchers.
Consequently, a complex sentence is opposed to a simple sentence as a unit of a polydispative unit of monopredicativity. Therefore, a complex sentence is a syntactic unit, the components of which are predicative units, united by a syntactic connection and syntactic relations.
So, in the formal-syntactic aspect, a complex sentence is a poly-preemptive unit, opposed to a simple sentence as a monopredictivity unit.
Ideally, a simple sentence is monopropositive : it contains a proposition, contained in a predicative combination of word forms or in one word form.