The sentence is the immediate integral unit of speech built up of words according to a definite syntactic pattern and distinguished by a contextually relevant communicative purpose.
The difference between the phrase and the sentence is fundamental: the phrase is a nominative unit which fulfils the function of polynomination denoting a complex referent (phenomenon of reality) analyzable into its component elements together with various relations between them; the sentence is a unit of predication which, naming a certain situational event, shows the relation of the denoted event towards reality.
The sentence is characterised by its specific category of predication which establishes the relation of the named phenomena to actual life. The general semantic category of modality is also defined by linguists as exposing the connection between the named objects and surrounding reality. However, modality, as different from predication, is not specifically confined to the sentence; this is a broader category revealed both in the grammatical elements of language and its lexical, purely nominative elements.
According to their structure, sentences are divided into simple and composite, the latter consisting of two more clauses. Simple sentences are usually classified into one-member and two-member. Another structural classification of simple sentences is their classification into complete and elliptical.
The composite sentence is built up by two or more predicative lines.
The compound sentence is based on parataxis, that is coordination. The complex sentence is based on hypotaxis, that is subordination.
Semi-composite sentences are sentences in which one predicative line is represented by a semi-predicative construction.