 The sentence is the highest ranking unit of syntax, since there are constructions that can hardly be defined as standard sentences. A distinction is drawn between two sentence types, major sentences and minor sentences. Major sentences have a fully developed subject predicate structure, as in a cat sat on the mat, where a cat is the subject and sat on the mat the predicate. The predicate in turn consists of a verb and an object. Major sentences allow operations such as substitution or transformation. Minor sentences, like Merry Christmas by contrast, behave as fixed units that cannot be rearranged. They often have no fully developed subject predicate structure. They do not allow the substitution of elements even if they are of the same type, and they cannot be transformed into different sentence types. Thus, in syntax, the focus is on major sentences. In present day English, there are four types of major sentences. Declarative sentences or statements, which generally have a subject that precedes the verb. Imperative sentences or commands that typically involve a verb in the base form and often have no overt subject. Interrogative sentences or questions, which either involve subject operator inversion or a WH element as a question word, and exclamatory sentences or exclamations that are introduced by a WH element but do not involve inversion. Note that major sentences can be elliptical, that is, some normally obligatory element of a grammatical sentence may be missing, but can be recovered.