The United States of America is a union of fifty states and is the third largest country in the world

Complex sentence
A complex sentence is a sentence that combines one independent clause with at least one dependent clause. A clause is a group of words that has both a subject and a verb. An independent clause forms a complete thought. A dependent clause, also called a subordinating clause, does not form a complete thought and is dependent upon an independent clause for meaning.

Examples of Complex Sentences

Here are some examples of complex sentences:
Although Denise had some doubts, she found the courses very useful.
The above example is a complex sentence: it has a dependent clause (a subject 'Denise' and a verb 'had'), followed by an independent clause (subject 'she' and a verb 'found') and forms a complete thought.
Computers have come a long way since they first came on the market.
This example begins with an independent clause (subject 'computers' and verb 'have come') that forms a complete thought, followed by a dependent clause (subject 'they' and a verb 'came').

Subordinating Conjunctions

In these examples, the dependent clauses begin with subordinating conjunctions or subordinators. Subordinators are conjunctions that indicate the subordinate or dependent role of the clause in the sentence.
The following words are some examples of subordinating conjunctions:

• 
  although
  
• 
  because
  
• 
  since
  
• 
  unless
  
• 
  when
  
• 
  whereas
  
• 
  which
  
• 
  while
  
• 
  who
  
• 
  whoever
  
• 
  whose
  

This is by no means a complete list.

Using Subordinate Clauses in Complex Sentences

In complex sentences, subordinate clauses function within the sentence as adjectives, adverbs, or nouns.
An adjective clause is a dependent clause that functions like an adjective; an adjective modifies or describes a noun. In this sentence:
The President chose people who would help him make good decisions,
the dependent clause 'who would help him make good decisions' functions as an adjective to modify 'people.'
An adverb clause is a dependent clause that functions like an adverb; an adverb modifies or describes a verb. In the example sentence:
When the food arrived, the children ate everything,
the subordinate clause 'when the food arrived' functions as an adverb to modify 'ate.'
A noun clause is a dependent clause that functions like a noun; a noun is a person, place, or thing. In the example sentence:
Whoever signs the lease is responsible for the rent,
the subordinate clause, 'whoever signs the lease,' serves as the subject of the sentence. This sentence is considered a complex sentence because it has both an independent clause (the entire sentence) and a dependent clause, 'whoever signs the lease.'