Sentence Structure

A sentence begins with a capital letter, ends with punctuation mark (or point), contains a subject and a verb (or verb phrase), and expresses a complete thought. It is an independent clause.
    Josie ran a marathon.

Simple Sentences
Simple sentences contain a subject and a verb and express a complete thought. It can have an object and modifiers. It is an independent clause.
    She ran.
    She (subject) ran (verb)—a complete thought.
    David threw a football.
    David (subject) threw (verb) a football (object).

Compound Sentences
A compound sentence contains two or more independent clauses (i.e., two or more sentences). They are usually combined by a comma with a coordinating conjunction (and, but, or, nor, for, yet, so) or a semicolon.
    David threw a football, and Elena caught the ball.
    The students waited for the bus, but the bus did not arrive on time.
    The students arrived at the bus station at 7 o’clock; that is, they arrived on time.

Complex Sentences
A complex sentence contains at least one independent clause (i.e., a sentence) and a least one dependent clause (i.e., a fragment). Dependent clauses usually can be identified by subordinating conjunctions like after, although, as, because, before, even though, if, since, though, unless, until, when, whenever, whereas, wherever, while, etc. 

If a sentence begins with a dependent clause, a comma follows the clause.
    Even though he enjoyed playing baseball, he was not very good at it.

If a dependent clause follows an independent clause (a sentence), a comma is usually omitted. At times, the flow of the sentence is better suited with the placement of a comma—this is left to the discretion of the writer or editor.
    He was not very good at baseball even though he enjoyed playing it.
    He was not very good at baseball, even though he enjoyed playing it.