If we say, The boys sing while the girls play, we have a complex sentence formed of the simple sentence, The boys sing, and the dependent clause, while the girls play.
406. Now let us sum up our definitions:
Every sentence must contain two parts, a subject and a predicate.
The subject of a sentence is that part about which something is said.
The predicate is that part which asserts something of the subject.
The simple subject of a sentence is a noun, or the word used in place of a noun, without modifiers.
The simple predicate is the verb or verb phrase without its modifiers.
The complete subject of a sentence is the simple subject with all of its modifiers.
The complete predicate of a sentence is the simple predicate with all of its modifiers.
A simple sentence is one which expresses a single statement, question or command.