Syntax as a part of grammatical description covers not only grammatical features of sentences, but also their referential meaning (semantic syntax). As a result variants of syntactic units which constitute their types are based not only upon the relations of sentence and phrase structures with grammatical meanings of this level, but also with the general meaning of the sentence. Details of the types of units of syntax discussed in the chapter dealing with general features of sentence and phrase. Now it is necessary to indicate the principles which may differentiate unites of the same class.
Sentences might be differentiated according to the form of the main element of the sentence and, namely, the logical subject. If the form of the logical subject does not depend upon the meaning of the verb or predicate, the sentences constitute the Nominative type. English sentences without exclusion belong to this type. Besides, sentences may be differentiated due to the number and syntactic quality of the elements they are composed of.
Phrases are differentiated according to the syntactical relations existing between their elements and, first of all, according to the presence or absence of dependent components.