What is a Sentence?
A sentence, in traditional grammar, a set of words so related as to express a complete thought. A sentence may contain one or more clauses. A simple sentence consists of one independent (main) clause; a compound sentence of two or more independent clauses; a complex sentence of one independent clause and one or more dependent clauses; and a compound-complex sentence of two or more independent clauses and one or more dependent clauses.
According to their use, sentences are classed as declarative (stating a fact), interrogative (asking a question), exclamatory (making a statement under the influence of strong feeling, or expressing a wish), and imperative (giving a command). A sentence usually has an expressed subject (a word or group of words referring to the person or thing that is talked about) and a predicate (a verb that, with its modifiers, asserts or predicates something about the subject). In an elliptical sentence, however, usually either the subject or the predicate is not expressed. This is especially common in an imperative or an exclamatory sentence (Do it! with you considered as understood; What a fine day! with it is understood), but there may also be ellipses in declarative and interrogative sentences (John, as an answer to the question Who is there? with is there understood; or What? with did you say or happened understood). Conventionalized exclamations or cries can also be considered as sentences (Oh, no! or Ouch!).