What Is A Sentence?

A sentence is a complete thought (can stand alone) and is defined as having subject (noun, pronoun) and predicate (verb) parts.

Declarative Sentences make statements and end with a period (.)
Imperative Sentences give commands and end with a period (.)
Interrogative Sentences ask questions ending with a question mark (?)
Exclamatory Sentences show strong feelings and end with an exclamation point (!)

Two words can make up a simple sentence:
Amber danced.
Johnny sings.
They cried.