A compound sentence combines two or more independent clauses, usually in order to compare, contrast, or emphasize their relationship in some way. The independent clauses should be statements of roughly equal importance. (See the examples below.)
Independent clauses are described here in depth, but all you really need to know about them for this discussion is that all clauses contain a subject and a predicate, but only an independent clause can stand alone as a sentence. You can tell if a clause can stand alone by applying the "I believe" test, which we also use to test for sentence fragments. Here's how it works:
Put the phrase I believe that in front of the sentence you wish to test. If it makes sense, your clause will stand alone as a sentence; if it doesn't make sense, your clause will not stand alone.
Example: William Shakespeare was born in 1564.
Test: I believe that William Shakespeare was born in 1564.
Conclusion: The clause is independent. It will stand alone.
Example: When William Shakespeare was born in 1564.
Test: I believe that when William Shakespeare was born in 1564.
Conclusion: This doesn't make sense. The clause cannot stand alone.
Example: Sir Francis Bacon was born in 1561.
Test: I believe that Sir Francis Bacon was born in 1561.
Conclusion: The clause will stand alone.
Example: Even though Sir Francis Bacon was born in 1561.
Test: I believe that even though Sir Francis Bacon was born in 1561.
Conclusion: This doesn't make sense. The clause cannot stand alone.
Independent clauses can be joined in three ways:
- with a comma and a coordinating conjunction (for, and, nor, but, or, yet, so -- remember FANBOYS)
- William Shakespeare was born in 1564, and Sir Francis Bacon was born in 1561.
- with a semicolon
- William Shakespeare was born in 1564; Sir Francis Bacon was born in 1561.
- with a colon
- William Shakespeare was a prolific author: he wrote approximately 37 plays.
Notice the difference between the semicolon and the colon. The semicolon separates two clauses that provide information that is closely related but DOES NOT define or expand on the same information in different words. The colon introduces a second clause that defines, expands on, or restates the information in the first clause.
If you want to separate independent clauses with a comma rather than a color or semicolon, always remember to combine it with a coordinating conjunction (for, and, nor, but, or, yet, so). It is considered incorrect to splice two independent clauses together with a comma alone. This error is called, appropriately enough, a comma splice.
Incorrect: William Shakespeare was born in 1564, Sir Francis Bacon was born in 1561.
Correct: William Shakespeare was born in 1564; Sir Francis Bacon was born in 1561.
Also be sure that you use only one of the FANBOYS conjunctions to support your comma. Other words, like however, nevertheless, then, thus, etc., may sound like conjunctions, but they are infact conjunctive adverbs.
Incorrect: William Shakespeare was born in 1564, however, Sir Francis Bacon was born in 1561.
Correct: William Shakespeare was born in 1564; however, Sir Francis Bacon was born in 1561.
You can read more about the conjunctive adverbs in the post entitled Making Sense of However.
Many authorities permit the comma splicing of very short independent clauses that have a strong parallel structure, such as "I came, I saw, I conquered."