A sentence that contains at least two independent clauses is a compound sentence. One way to connect the independent clauses is by using a conjunctive adverb; another way is by using a coordinating conjunction.

There are seven coordinating conjunctions: for, and, nor, but, or, yet, and so. As a group, they form the “fanboys,” an acronym that aids their recall.

Whenever a coordinating conjunction connects two independent clauses, a comma must be inserted before the conjunction.

Example: Jill ate the hamburger, for she was hungry.

Note: In the above example, “for” means “because.”

Is the following a compound sentence? Is the sentence punctuated correctly?

* The Mets released Luis Castillo on Friday but they didn’t release Oliver Perez until Monday.

It’s a compound sentence; however, it’s not punctuated correctly. It needs a comma before “but.”