The concepts of obligatory and optional in the syntax are used within the framework of the compatibility characteristic, which is determined by the valence properties of words. Conformity is not related to the position of words in the structure of the sentence, therefore it manifests itself in all syntactic structures, for example, “the girl, playing the violin”, “playing the violin, the girl went to the window”, etc. At the same time, many scientists point out that valence compatibility is provided by the lexical semantics of words (Плоткин 2021: 67; Прокопович 1955: 213; Чеснокова 2003: 89). At the same time, this type of compatibility in science is called grammatical compatibility, and the scientists describe its obligatory and optional nature as structural one (Адмони 1958: 73; Ломтев 1961: 17; Иванникова 1965: 19; Почепцов 1968: 92). However, in recent years, in most scientific studies, doubts have been expressed about the advisability of classifying valence compatibility as categories of syntactic structure, since valence compatibility appears as a manifestation of the lexical properties of words (Булгакова 1971: 88; Левицкий 2019: 156). Nevertheless, valence compatibility as a structural compatibility can be explained by the fact that its implementation is always associated with the expression of syntactic relations, and, therefore, with the construction and development of syntactic constructions, which allows us to consider justified the use of the term “structural obligation and optional” in relation to valence compatibility.

We believe that the objections widespread in the scientific expert community are caused by the fact that experts do not completely agree with the description of grammatical compatibility existing in linguistics. The problem is that until recently in the scientific literature such phenomena as grammatical compatibility were almost not differentiated, since scientists focused on valence compatibility and its properties, without highlighting other types of syntactic compatibility. Moreover, in linguistics, compatibility was usually determined by structural categories. At the same time, the delimitation of constructive and communicative syntactic aspects necessitates changing views on such a syntactic phenomenon as the compatibility of syntactic units. Separate works of linguists are devoted to attempts to designate along with constructive and optional compulsory communication in a communicative sense (Почепцов 1971: 63; Чеснокова 1972: 151). On this basis, analyzing the nature of compatibility, they distinguish between the concepts of mandatory and optional compatibility in the communicative and constructive aspects.

In a communicative sense, each component of a sentence that carries information (reporting a certain phenomenon of objective reality) becomes mandatory in a particular sentence: it cannot be removed without communicative and semantic damage to the statement as an act of communication. In particular, in the sentence “Our house was the last but one on the edge, and there began a deserted field, in some places overgrown with bushes” (I.S. Turgenev) all components are communicatively required. Based on this, at first glance, it may seem that this approach makes it impossible to designate optional components in a communicative sense, since each component of the sentence denotes something, and, therefore, is mandatory. At the same time, it is important to pay attention to the fact that communicative faculty can be expressed by information presented in a certain component of the utterance in a different way. From a communicative point of view, faculty becomes a manifestation of redundancy in the language. The component of the utterance, the meaning of which is included in the meaning of another component, is communicatively optional. Here are some examples: “He rubs his sleepy eyes, examines us one by one in surprise, as if not believing that they returned alive” (G.A. Fedoseev “Evil spirit Yambuy”); “You grow up big and go” (V.A. Soloukhin “Vladimir country roads”). In these examples, the words “alive” and “big” are optional in a communicative sense, since their removal from sentences does not affect the communicative semantics of statements (Павлов 1973: 87; Кучерова 2020: 57).

In the process of designating constructively obligatory and optional compatibility versus communicative obligatory and optional, it is not possible to limit the area of constructive compatibility exclusively to the phenomena of valence compatibility. Constructive compatibility indicates two types of different phenomena: 1) valence (structural) compatibility; 2) compatibility within the syntactic model (model compatibility). Structural obligatory and optional is the expression of the internal properties of words according to their lexical and grammatical meanings. Obligatory or optional model is associated with the participation or non-participation of a component in the creation of a specific syntactic model. The model is an abstract image that allows one to represent in the most general form the formal characteristics and grammatical content of a certain syntactic structure. The model manifests itself in various constructions, but these concepts are not identical. In particular, the structure may contain components not represented in the original (nuclear) model. For example, in the construction “In summer, children sleep on the porch”, except for the components providing the predicative model “children sleep”, there are components “in summer” and “on the porch” that are not involved in the implementation of this model. On this basis, we introduce the concepts of model-obligatory and model-optional components of the structure. The former are components that are necessary for the implementation of a specific model, the latter do not take part in this process, and they only expand the structure by adding various units of information to it. We suggest to consider these provisions regarding a number of proposal and non-proposal constructions. In this regard, the idea of T.P. Lomtev that the sentence model is the remainder after the exclusion of its individual components from the sentence in terms of sound and concreteness of meaning (Кочетова 2019: 118).

All propositional constructions implement a specific sentence model with a specific structural type of sentence. In particular, if we consider the model of a nominative one-component sentence in the Russian language, which is one of the structural types of a sentence, then it is realized by means of a one-term construction, for example, «Осень». In terms of constructing this model, the component «осень» is a model obligatory one. However, in the sentence «Поздняя осень», which is implemented according to the same model, a model obligatory component «осень» and model optional component «поздняя» with additional information are presented. In English, the construction “The rooks flew south” implements a two-part sentence model, while the components “rooks” and “flew” are model obligatory ones, and the component «на юг» is the model optional component, however, this conclusion does not lead to automatic recognition of this component as communicatively optional one. In model constructions with a double attributive-adverbial relation, for example, “They saw him upset”, “He worked sick”, “He will be the first to answer”, “Turned to him first” becomes unimportant whether this model is embodied through propositional or non-propositional structures, since three elements appear to be model obligatory: name – verb – name (verbs – noun qualifier). In this regard, in the constructions “He worked sick today”, “He is the first to answer at the exam”, “Suddenly see him upset” the components “today”, “at the exam” and “suddenly” are the model obligatory.

It is useful to pay attention to the fact that in a sentence as a communicative act; two or more models can be realized simultaneously, which have an unequal grammatical nature. In other words, each syntactic structure that has, in addition to components that implement a specific model, other components outside this model, acts as a union of several models, where one of the models becomes the main (or leading) one for such a structure. Here is an example of a proposal with multiple models: “In the early morning children go to school from home”, it includes: 1) sentence model “children go”, 2) attributive model “in the early morning”, 3) adverbial temporal model “in the early morning go”, 4) adverbial local model “go to school from home”. In essence, the division presented above is similar to the division of the text into simple phrases, which is quite natural, because all phrases are special constructions, formed taking into account the form and content, which becomes the implementation of a specific syntactic model. It suggests that all the sentence’s components are model binding, since they are necessary within the framework of a certain model implemented in the proposal. V.V. Bulgakova noted in this regard that at the level of syntactic structures there is no optional compatibility, since, from the point of view of syntax, the adverbial positions of this structure are obligatory. If in terms of meaning’s completeness a number of positions can be excluded, then syntactically this will lead to a change in the structure, in other words, one model will be replaced by another (Brown, Miller 2017: 78; Curme 2020: 102). All of this points to the fact that the sentences “The rooks flew south” and “The rooks flew south a long time ago” refer to different models, and all the components of these sentences are obligatory in terms of building a particular model. At the same time, this conclusion is valid only in relation to the most general understanding of the syntactic structure, since the qualitative difference in structures is not taken into account.

When an analysis is carried out to determine the features of the structure of a particular construction, in order to establish the models that are implemented in the construction, it becomes necessary to indicate not only the models, but also the components that are required for the implementation of all types of model. Therefore, it is required to designate the optional components of all models. The results of this analysis make it obvious that a component can be model obligatory in one type of model and optional in another one. In particular, in the previously mentioned sentence “In the early morning children go to school from home” components “children” and “go” become obligatory for the predicative model, and all the rest are optional components, since it is the named components in their combination that provide predicativity. In this case, for the attributive model the required component is “in the early morning”. Others are optional components: the adverbial model implies obligatory components “go” and “to school”, etc. In this regard, we can conclude that the model obligatory and optional should be considered, first of all, in relation to simple models that are not subject to subsequent division into their constituent syntactic models. When simple models are combined into a single structure, this structure is perceived as a complex syntactic construction, when a simple sentence, for example, “The rooks flew south” refers to complex syntactic constructions created by combining two simple models: predicative and adverbial local ones. The approach we have described to the structure of syntactic constructions allows us to reveal the patterns of combining simple structures into complex ones.

In the process of establishing the categories of obligatory and optional, it is essential to take into account the qualitative difference in linguistic facts, regardless of the aspect in which these categories are determined, but all aspects have their own specifics for such categories. Structurally obligatory components (valence compatibility) are based on the semantics of words and are associated with their meanings related to a specific semantic group. Structurally optional components, on the other hand, are only allowed by the semantics of words, but do not depend on it. Communicative optionality (combinability of words in the communication aspect) is possible only in conditions of certain meanings of the combined words. In particular, in the sentences “He hastily bit the snow several times with his teeth...” (А.I. Kuprin) and “He hastily grabbed the snow with his teeth” the component “with his teeth” with the verb “bit” is communicatively optional, since the specified verb has already conveyed all the necessary information; however, the same component in the verb “grabbed” is communicatively obligatory, it already carries new information.

The model obligation of the components is established by their participation in the implementation of a specific qualitatively specific model. Model optionality can be defined solely in relation to a model of a particular type; it becomes a feature of structural components that do not participate in the implementation of the model. In particular, in the construction “a very interesting book” the component “very” is optional in the attributive model, but in the construction “the girl took the book” the component “the book” is optional in the predictive model.

Thus, despite the fact that the establishment of mandatory and optional in the syntax is important for understanding the structure of a sentence and its analysis, the allocation of model-mandatory and optional components in the structure of a sentence is a particular issue of the problem of model obligation and optional in general, since the definition of these categories cannot be limited by any one model. It seems to be a common feature of the syntax of structures that underlies the creation of any type structures. In this regard, if we recognize the presence in the language different types of propositional and non-propositional models, as well as single out these models in the form of specific constructions, then model obligatory and optional components inevitably arise, regardless of whether these categories are theoretically isolated as syntactic concepts or not.