Comma in Compound Sentences

A compound sentence refers to a sentence made up of two independent clauses (or complete sentences) connected to one another with a coordinating conjunction. Coordinating conjunctions are easy to remember if you think of the words "FAN BOYS":
  • For
  • And
  • Nor
  • But
  • Or
  • Yet
  • So

The following are examples of compound sentences:
  • Johan waited for the train, but the train was late.
  • Mariam and Sakinah arrived at the bus station before noon, and they left on the bus before I arrived.
  • Mariam and Sakinah left on the bus before I arrived, so I did not see them at the bus station.

USE A COMMA BEFORE A COORDINATING CONJUNCTION THAT JOINS TWO INDEPENDENT CLAUSES. HOWEVER, COMMA IS NOT NECESSARY FOR SHORT CLAUSES. 
If the two "sentences" (known as independent clauses) are very short, it is acceptable – for style purposes – to omit the comma.
  • Johan joined the Army and Daud joined the Marines.
  • Johan joined the Army, and Daud joined the Marines.