Simple, Compound, and Complex Sentences

There are three types of sentences:

Simple Sentences: The simplest of simple sentences have a simple subject (one actor or set of actors) and a simple predicate (one action).  

The dog ate.

The mouse ran.

Simple sentences can have compound subjects (two actors joined with a conjunction):

The dog and the cat ate.

The mouse and the monkey ran.

Or, they can have compound predicates (two actions joined with a conjunction):

The dog ate and slept.

The mouse ran and squeaked.

Simple sentences are also known as independent clauses.  Independent clauses can stand by themselves (as simple sentences), can be combined with other independent clauses (see compound sentences below), and can be combined with dependent clauses (see complex sentences below).

Compound Sentences: When two simple sentences (or independent clauses) are joined together, they form a compound sentence.  You can join independent clauses with a coordinating conjunction (coordinating conjunctions are: for, and, nor, but, or, yet, so; you can remember these conjunctions with the acronym FANBOYS).  The following are compound sentences joined with a coordinating conjunction:

 The dog ate, and the mouse ran.

The dog ate, but the mouse ran.

The dog ate and slept, and the mouse ran and squeaked.

You can also join independent clauses and create a compound sentence using a semicolon (don’t use a coordinating conjunction if you use a semicolon):

The dog ate; the mouse ran.

The dog ate; the mouse ran.

The dog ate and slept; the mouse ran and squeaked.

You cannot just run the independent clauses together or join them with a comma.  If you do, you will create a run-on sentence (if you use a comma, it’s a special type of run-on called a “comma splice”).  Both of the following examples are run-ons:

The dog ate the mouse ran.

The dog ate, the mouse ran.

Complex Sentences:  When you add a dependent clause to an independent clause, you’ve formed a complex sentence.  A dependent clause is a clause that cannot stand on its own (if you write one on its own, it will be a sentence fragment).  Dependent clauses include phrases that start with subordinating conjunctions such as: because, while, since, although, even though, in spite of, despite, after, before, although, or relative pronouns such as: that, which, or who.  The following sentences are compound sentences (the dependent clauses are bolded):

While you were sleeping, the dog ate.

The mouse ran although there was no where to go.

The dog ate the mouse who was not happy about it.

None of those dependent clauses could stand alone (they would be fragments):

While you were sleeping

Although there was no where to go

Who was not happy about it

A note about commas: When you write a complex sentence in which the dependent clause comes first, you must put a comma after the dependent clause.  If the dependent clause comes after the independent clause, only use a comma if the dependent clause is non-essential (could be left off without really changing the meaning of the sentence).

If the dog liked the cat, they would hang out together.

The dog and the cat fight when they are together.

Compound-Complex Sentences:

You can combine the above types of sentences for compound-complex sentences:

While you were sleeping, the dog ate and the mouse squeaked.

Or even a compound-complex sentence with a compound subject and predicate:

The mouse and the rat ran, and the dog scampered and skittered, although there was no way to escape.

Good writers use all kinds of sentences in their writing.