© Nani, 2013

31. When Do I Use Commas?

A. To join independent clauses into compound sentences

I. An independent clause is a sentence that acts as a single, complete thought.
 

•  Example: “The ancient Mesopotamians developed a system of canals.”
•  Example: “Mesopotamian canals created severely salinized soil.”

  Both of these sentences, while related in theme, function as complete, independent thoughts. They are independent clauses.
 
II. A dependent clause is a sentence that relies in some meaningful way on another clause for clarity.
 

•  Example: “To irrigate their fields and crops.”
•  Example: “Deposited extra sediment that was not properly drained.”

  These clauses depend on another statement for clarity and meaning. They are dependent clauses.
 
III. A compound sentence consists of two independent clauses joined by a comma and linked by a conjunction. These parts of the sentence can be seen as separate but still function as one complete thought. If divided, they risk becoming two choppy sentences, so the comma and conjunction are vital in avoiding that problem.
 

•  Example: “The ancient Mesopotamians developed a system of canals, but these canals created severely salinized soil.”
•  Example: “The ancient Mesopotamians developed a system of canals, which led to severely salinized soil.”

  In the second example, the subject is removed for simplicity because it has now been compounded with that of the first clause. Either example is correct.
 
IV. To test if a sentence has two independent clauses that should be joined by a comma and a conjunction, identify the main thought in each part of the sentence and determine if it can stand alone or if it needs the information in the adjacent clause for clarity and meaning. Generally, you will end up with two clauses, or parts, of the sentence. If the second clause depends in some way on the first clause (usually the subject or the verb), it is not a complete (and therefore independent) thought. In this case, there is no need for a comma before the conjunction.
 

•  Example: “The ancient Mesopotamians developed a system of canals in order to irrigate their fields and crops.”
•  Example: “Mesopotamian canals created severely salinized soil because they brought in extra sediment that was not properly drained.”

  If the first clause depends in some way on the second clause, it acts as an introductory clause and should be separated from the main sentence by a comma (see next section).
 

•  Example: “In order to irrigate their fields and crops, the ancient Mesopotamians developed a system of canals.”

 

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