What is a missing comma with compound sentence error?

A compound sentence is a sentence with at least two independent clauses, or parts that can stand alone as separate sentences. For example, "A dog barks, but a cat meows." Note that independent clauses in compound sentences are joined with coordinating conjunctions, or FANBOYS (for, and, nor, but, or, yet, and so). The rule is that you must place a comma before this conjunction. If you do not include one, you have written a fused or run-on sentence.

Examples of missing commas with compound sentences

• A dog barks but a cat meows.
• The mayor's office is downtown and the police department is next to it.
• Punctuation can be tricky yet it is worth the effort to learn it.

Fixing run-on sentences

Fixing this error is simple--just put a comma in front of the conjunction.

Examples of fixed run-on sentences

Notice how the sentences above have been punctuated in the following examples.

• A dog barks, but a cat meows.
• The mayor's office is downtown, and the police department is next to it.
• Punctuation can be tricky, yet it is worth the effort to learn it.

Learn more under "commas with two independent clauses" here.