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

2.2  A complex sentence as a special unit of syntax.
As you know, the basic unit of traditional syntax is a sentence. 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. "A complex sentence is the union of several sentences with the help of one or another syntactic medium into a grammatical whole, serving as an expression of the finished thought. Like any sentences, a complex sentence is generally characterized by the intonation of the end. The separate sentences that are part of a complex sentences do not have independence and completeness and are interrelated and complementary elements of a whole complex sentences"[20, 172]
In structure, the simple sentence is opposed by a complex sentence. The difference between them is that the first is monopredicative (that is, the predicative relation characterizing the reciprocal relationship of the subject and the predicate is represented in the sentence once), while the second is polypredicative .The components of a complex sentence by some researchers are also considered as suggestions. We can argue with this point of view, because, in our opinion, the subordinate is not a sentence, if only because it is devoid of independent communicative significance. It is used in the process and for the purposes of verbal communication only as a component of a greater syntactic unit - a complex sentence. Even parts of the compound sentence are inadequate as units of communication. Often, their mutual relations are related to the semantic relationships of the cause-effect, the temporary organization, etc., and breaking them, isolating each of the parts of the compound sentence into an independent sentence means weakening or breaking the syntactic and semantic links that exist between them. In addition, the non-finite parts of the compound sentence can be syntactically communicated with oneself similarly and intonationally. Being isolated from the rest of the complex sentence, such constructions turn out to be intonationally different from the sentence.
Polypredicative complex sentence means not just the representation in it of multiple predicative relations. In the sentence:
He waved his hand in the direction of the house and was silent (A. Huxley)
A predicative relation occurs twice in connection with “waved his hand” and etc. and in connection with “was silent”. Each of these groups is characterized by predicative connection with “he”, but there is no complex sentence here. Important
Therefore, to refine the above characteristic of a complex sentence, pointing out that in a complex sentence there are several predicative centers consisting of the subject and the predicate.
Thus, "a complex sentence is the structural and semantic unity of two or more syntactic constructions, each with its predicative center, which is formed on the basis of a syntactic connection and used in speech communication as a single- entry unit with a simple sentence"[11, 382]
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".
Sentences in English are divided by their structure as simple (Simple Sentences) and complex (Complex Sentences) sentences:
I want to go to London (simple)
I never thought I would go to watch a football match of a team I love (complex) 
Individual sentences that are part of a complex one, on the one hand, are similar to simple sentences, on the other, they differ from them. The similarity of individual sentences, which are elements of a complex sentence, with simple sentences, consists in their grammatical structure: both have the main terms expressing predicativity - the subject and predicate in two-part sentences or the main member in single-member sentences, and may have secondary members, which are tied into a common continuous chain
Subordinate word combinations, the starting point of which are the main members. As in simple sentences, each such subordinate chain is independent of the same subordinate chains of other sentences. And in complex there are as many separate sentences as there are such unities with their main members in it. Over the past 70-80 years a number of researchers have expressed in various works on the syntax of a complex sentence a doubt about the expediency of preserving in science the traditional scheme of dividing the links between parts of a complex sentence into coherent and subordinate ones. "Our view on the essence of the work and submission, - wrote V.I. Zhelvis , - is stated in the article "On the question of the types of syntactic connection between the parts of a complex union sentences". We believe that composition and submission objectively exist in the language and represent semantic categories that find a definite formal expression in the language. The semantic distinction between composition and submission consists, in our opinion, in the fact that when composing a part of a complex sentence, united by an alliance, they are equal, while under subordination one part is dependent on the other, in one way or another serves it. In both cases, parts of the sentence are very closely related to each other"[23, 254]
What are the compositional and subordinate connections in a complex sentence? Under the creative connection of the two parts of the sentence, according to Gelwis , "one must understand such a connection in which these parts equally depend on each other in the semantic, grammatical and rhythmomelodic relationship. With subordination, one of the parts of the complex sentence (the main part) in the semantic, grammatical and partly rhythm-melodic relation is more independent than the second (accessory) part"[22, 344]
In general, subordination is a case of a closer connection between separate parts of the expressed thought than the composition.
The combination of sentences within the complex and the establishment of various relations between them, according to AN Gvozdev , is carried out by a number of syntactic means. These include: intonation, conjunctions, union words, correlative pronominal words, order of sentences, use of times and inclinations, lexical and phraseological elements.[20, 175]
Proceeding from the above, it should be concluded that the study of a complex sentence within the framework of modern linguistic science is a constantly evolving phenomenon, since the communicative function of the sentence underlies the development of the entire human civilization.