A third common type of sentence is a complex sentence. This type has one independent clause and one or more dependent clauses.

The similarity between an independent clause and a dependent clause is that each is a group of words that has a subject and a verb. The difference between the two clauses is that an independent clause makes a complete statement while a dependent clause does not make a complete statement. The reason for this difference is the beginning word of the dependent clause.
        Example: You have worked hard lately. (Independent)
Because you have worked hard lately, (Dependent)

Complex Sentence Patterns
A complex sentence can begin with either a dependent clause or an independent clause. Notice the use of a comma in one pattern but not the other. For variety, use both patterns in your writing.
In the following complex sentence patterns, IC = Independent Clause and DC = Dependent Clause. In the examples, dependent clauses are enclosed in parentheses ( ).

1. DC, IC Example: (When I heard of the accident,) I panicked.
2. IC DC Example: I panicked (when I heard of the accident.)

The word that begins a dependent clause is called a subordinating conjunction. These conjunctions have the effect of making the words that follow them dependent on the main part (the independent clause) of the sentence. If the subordinating conjunction is taken away, the remaining words will then express a complete thought. Omit when in the two examples above and you will see how this works.

Dependent clauses that answer when? where? why? and how? are called adverb (dependent) clauses. Adverb clauses start with a subordinating conjunction, many of which are in the following list.

Subordinating Conjunctions
after although as as if
as long as as soon as as though because
before even if even though if
in order that just as once provided
provided that rather than since so that
than unless until when
whenever where wherever while

Another group of small words called relative pronouns make it possible to have a third complex sentence pattern.

Relative Pronouns
who whose whom that
which whoever whomever whichever

3. IC DC (IC) Example: The pilot (who flew the vintage airplane) did a death spiral.
At first glance, it seems that the pattern has two independent clauses. However, in this pattern, the independent clause is interrupted by the dependent clause. The first IC stands for the subject of the independent clause while the second (IC) contains the verb of the independent clause. There is only one independent clause. The interrupting dependent clause is called an adjective (dependent) clause.

When you don’t specifically know who the subject is in the independent clause, do not use commas around the dependent clause. The dependent clause pinpoints the subject precisely and so is needed in the sentence.
When the subject is clearly identified in the independent clause, do use commas around the dependent clause because it will just be giving extra, but not essential, information.

This situation creates a pattern variation: IC, DC, (IC)
Example: Riley Scott, (who flew the vintage airplane), did a death spiral.