What is the difference between a compound sentence and a complex sentence?

A compound sentence consists of two independent clauses joined by a coordinating conjunction. A complex sentence is constructed using a dependent clause, beginning with a subordinating conjunction, and an independent clause.

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Before we can talk about the difference between compound and complex sentences, we have to discuss the difference between independent and dependent clauses. An independent clause can stand alone as a complete thought. For example, “The cat snoozed on the couch” is an independent clause. It has a subject and a verb and presents a full idea. However, notice the difference in this clause: “Because the cat snoozed on the couch.” It, too, contains a subject and a verb, but it is not a complete thought. Readers naturally ask, “Because why?” This is a dependent clause. Words like “because” are called subordinating conjunctions; they make a clause dependent upon another clause.

With that foundation set in place, we can now discuss compound and complex sentences. A compound sentence joins together two or more independent clauses using coordinating conjunctions (and, but, yet, nor, so, or, for). For instance, “The cat snoozed on the couch, and she left a pile of fur behind” is a compound sentence made up of two independent clauses—“the cat snoozed on the couch” and “she left a pile of fur behind”—joined by a comma and the coordinating conjunction “and.”

A complex sentence, on the other hand, joins together one dependent clause and one independent clause linked by a subordinating conjunction. The dependent clause may appear first, as in: “Because the cat snoozed on the couch, she was awake all night.” Notice the subordinating clause “because” that begins the dependent clause. Since the dependent clause comes first, it is followed by a comma and then the independent clause “she was awake all night.” We might also write: “The cat was awake all night because she snoozed on the couch all day.” Notice that here we begin with the independent clause followed by the dependent clause. There is no comma between the clauses when the dependent clause appears last.

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To define types of sentences, you must first understand clauses. An independent clause is also known as a complete sentence. It has a subject and a predicate and expresses a full thought. An independent clause can be short: Juan coughed.

Juan is the subject, and coughed is the predicate. There isn't any additional information, but this is a core independent clause. Independent clauses can also be much longer: My best friend Juan loudly coughed in math class.

This independent clause contains modifying information, but it's still centered around one core subject and predicate.

A dependent clause begins with a subordinating conjunction. It contains a subject and a predicate but cannot stand alone as a sentence. It might look like this: Since he had just eaten a hot pepper.

Since is the subordinating conjunction in this sentence. He is the subject and had eaten is the predicate, but this series of words cannot stand as one complete thought because of the implications of that subordinating conjunction.

A compound sentence is created by combining two independent clauses with a coordinating conjunction: Juan coughed, and everyone turned to stare at him.

In this sentence, our original independent clause is linked to a second independent clause by and, which is a coordinating conjunction.

A complex sentence is created by joining a dependent clause to an independent clause: Since he had just eaten a hot pepper, Juan coughed.

When the dependent clause comes first, you'll need to use a comma before the independent clause. However, if the independent clause comes first, you should omit the comma: Juan coughed since he had just eaten a hot pepper.

Both of these, regardless of whether the independent clause comes first or second, are complex sentences.

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Just to add a little more detail about compound sentences, there may be two or more independent clauses:

"I was very thirsty, and I wanted something refreshing; the lemonade stand a block down the street seemed more and more enticing."

In this case, there are three independent clauses.  Two of them are separated by the comma and the conjunction and.  The third is set off by the semicolon, which can also be used to separate independent clauses.

Variety is important, and good writers learn to manage their sentence structure using any combination/number of independent and dependent clauses.  Just be sure to use the tools to separate them correctly, and strive for clarity.  

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The difference between a compound sentence and complex sentence is that a compound sentence has two independent clauses and a complex sentence has one independent clause and one dependent clause.

A sentence is a group of words that has a subject and a predicate.  A subject, centered on a noun, is what the sentence is about.  A predicate, centered on a verb, tells what happens to the subject.  It tells what the subject does or is.

The dog jumped over the fence.

In this sentence, the bold part is the subject, and the second part is the predicate.

Sentences are made up of clauses, or groups of words.  An independent clause, like the one above, can exist independently and makes complete sense on its own.  A subordinate or dependent clause, on the other hand, requires an independent clause.  The italicized portion of this sentence is a subordinate clause.

Sentence #1:  Because it was chasing a rabbit, the dog jumped over the fence.

Let’s look at that italicized portion without the independent clause holding it up.

Because it was chasing a rabbit.

This is not a sentence by itself.  It needs the rest of the sentence to be a complete thought.  That makes our rabbit sentence (Sentence #1) a complex sentence.

A compound sentence, on the other hand, is made up of two independent clauses.  If we remove the word “because,” the sentence becomes a complete sentence.

It was chasing a rabbit.

Compound sentences are usually combined with a coordinating conjunction such as for, and, nor, but, or, yet, and so.

Sentence #2:  It was chasing a rabbit, and the dog jumped over the fence.

In this sentence, both clauses are independent clauses.  This means that each clause (It was chasing a rabbit) and (the dog jumped over the fence) are both complete sentences themselves.  Combined together they become a compound sentence.

When you are writing, you want to vary the sentence structure you use to make your writing more interesting and sophisticated.  Make sure you write in complete sentences, use clauses correctly, and keep your writing fresh.

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