A compound sentence is a sentence that has two or more parts that are independent clauses, i.e., each can stand alone as a full sentence with a subject, a verb and an object.
These simple sentences in a compound sentence are grammatically equal structures, separated by a comma, but joined by a coordinating conjunction: for, and, nor, but, or, yet, and so. (Taken together, the first letters spell FANBOYS.) The comma goes before the coordinating conjunction, not after it (even though that might be how you pause when you speak the words).
Bad Example: We left for the airport early but, we still missed the plane.
Good Example: We left for the airport early, but we still missed the plane.
Compound sentences can also use the formulaic structure of correlatives: either-or, neither-nor, not only-but also, both-and. A comma is used to separate both complete clauses.
Example: Not only was the well drilled in record time, but it was also completed in a single day.
Under certain circumstances, the comma between independent clauses in a compound sentence can be – and sometimes should be – left out:
- If both independent clauses are short and very closely related, especially if the subject of both clauses is the same.
Example: I saw the movie and I liked it.
- If only the first clause is short, and the two clauses are closely related, especially if the subject of both clauses is the same.
Example: I got in line early so I could get front-row seats to the concert.