1. the subject of theoretical grammar its relations to other branches of linguistics

Major and minor sentences

A major sentence is a regular sentence; it has a subject and a predicate. For example: "I have a ball." In this sentence one can change the persons: "We have a ball." However, a minor sentence is an irregular type of sentence. It does not contain a finite verb. For example, "Mary!" "Yes." "Coffee." etc. Other examples of minor sentences are headings (e.g. the heading of this entry), stereotyped expressions ("Hello!"), emotional expressions ("Wow!"), proverbs, etc. This can also include nominal sentences like "The more, the merrier". These do not contain verbs in order to intensify the meaning around the nouns and are normally found in poetry and catchphrases.

Sentences that comprise a single word are called word sentences, and the words themselves sentence words.

42 - The main characteristics of the sentence
An important structural feature of the sentence is its entirety, that is, no word of the given sentence can be the head or a dependent element relative to words that stand outside this sentence.
So, the sentence can be defined as an immediate integral unit used in speech communication, built up of words according to a definite syntactic pattern and characterized by predication. It possesses the following properties:
The sentence as a linguistic expression of extralinguistic reality must be actualized. Actualization of the sentence content makes predicativity an inseparable property of every sentence.
The sentence, just like any other meaningful language unit, has a form. Every sentence has an intonation pattern.
The sentence occupies the highest hierarchical position relative to other structural language units since the final purpose of all structural language units is to build sentences. Unlike the sentence, the text does not have accurate and unambiguous structural characteristics. There are no universal structural schemes of the text. None of semantico-structural means used to join sentences is specific to the text. Therefore, the text cannot be considered a structural language unit.