Compound sentences consist of two or more independent clauses. Unlike sentences whose independent clauses are followed by phrases, compound sentences require a comma between each independent clause. Here's how you can know whether a sentence with multiple clauses or phrases requires a comma:

1. Locate the part of the sentence that follows the coordinating conjunction. The coordinating conjunctions are and, but, or, nor, so, for, and yet.

   1. The couple built a log cabin, and they lived happily ever after.

   2. The couple built a log cabin and lived happily ever after.

   In example a, "they lived happily ever after" is an independent clause and a complete sentence. "They" is the subject and "lived" is the verb. In example b, "lived happily ever after" is not a complete sentence because there is no subject.

2. If the part following the conjunction forms a complete sentence, as in example a, insert a comma before the conjunction. If the part following the conjunction does not form a complete sentence, as in example b, do not insert a comma.

Remember: Think of independent clauses as mini-sentences. Place a comma before the conjunction when it separates two mini-sentences. If there is only one independent clause, omit the comma.

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