Before we start discussing grammatical features of sentences we have to define their specific property which differentiates them from other units. In the linguistic theory we can find a great number of different definitions of this unit, but all of them have a common feature. That feature is the idea of completeness. A sentence can be defined as a completed structure, or as a completed representation of a thought, or as a completed and independent unit of speech. All of these definitions can be criticised because we can only say that the structure is not complete, or the thought is not fully represented, or that the unit which we call a sentence is not quite independent in speech. It makes us think that completeness here means only that the unit we call a sentence is presented as complete. If so, there should be certain indicators of the fact that the speaker presented us with what he thinks is enough, and thus complete. To find these indicators we should recall the function of the sentence
The function of the sentence is to model some fragment of the world. This model should be referred to the original, to a fragment of the world. It means that a sentence is to be referred to the reality. This property of sentence was first proposed and described be V.V. Vinogradov in late 30-s. V.V.Vinogradov named this feature predicativity and showed that predicativity means reference of the contents of the sentence to the reality indicating the time of it relative to the moment of speech, the degree of probability of the fact described in the sentence, and its relation to the participants of communication. If we know when, how true and whom or what about is a word or a group of words, then this word or group of words presents a complete idea, and is a sentence. These three features of predicativity may have their grammatical representation. The when-component of predicativity is known as the temporal system and is shown by the tense. The how true-component of predicativity is represented by modality and shown by the mood or modal elements. The who- or what about-component shows whether the statement is made about the speaker or the listener or somebody or something else and is formalised as a personal component of predicativity.