sentence

A sentence is the largest grammatical unit in language. It communicates a complete thought—an assertion, question, command, or exclamation. In general, assertions and questions—the overwhelming majority of sentences—require a subject and a verb, put together in a way that can stand alone, resulting in what is called an independent clause (see main clause): He kicked the ball is a sentence. After he kicked the ball is not a sentence; instead it is a dependent clause (see subordinate clause). Even though it has a subject and a verb, it needs to be connected to something in order to complete the assertion: After he kicked the ball, he fell down; or He fell down after he kicked the ball. In the case of commands, the subject need not be written because “you” is understood: Go home! means You go home! And exclamations clearly express excitement, alarm, anger, or the like with no need for either a subject or a verb: Wow! Gadzooks! Ouch!
In everyday speech we routinely use phrases or clauses that would not make a complete sentence—so-called sentence fragments—because the conversation or the circumstances make the meaning clear. For example, we might answer a question like “Where did you go?” with “To the store,” or “Why can’t I stay out till midnight?” with “Because I say so,” or “What are you doing?” with “Trying to fix this toaster,” instead of “I went to the store,” “You can't stay out that late because I say so,” or “I am trying to fix this toaster.” In written dialogue sentence fragments are perfectly acceptable. They would generally be regarded as sentences simply because they begin with a capital letter and end with a suitable punctuation mark. But they are not sentences in a strict grammatical sense. And as a rule, sentence fragments are frowned upon in formal or expository writing. They can be useful—indeed, powerful—but in such writing they are effective only if used sparingly, in order to achieve a deliberate special effect: We will not give up fighting for this cause. Not now. Not ever.