The use or non-use of a comma is important in sentences containing more than one clause.
A clause is a group of words expressing a thought and having a subject and a predicate (a verb relating to the subject): “He (subject) ran (predicate) to the store.”
An independent clause is one which expresses a complete thought and can stand alone: “She plays the violin well.” “Mary takes music lessons.” Both thoughts are complete and need no further help to be understood.
A dependent clause is one which has a subject and predicate but does not express a complete thought: “Mary who takes lessons. . .” and “Because she plays the violin well. . .” are dependent clauses because each needs something more to complete its thought.
We could use the two independent clauses in two separate sentences without joining them; it would be proper but jerky. “Mary takes music lessons. She plays the violin well.” By combining them with a conjunction and a comma into a single sentence, there is a more comfortable flow: “Mary takes music lessons, and she plays the violin well.” This is a compound sentence (two or more independent clauses), and as a general rule the comma always proceeds the conjunction in a compound sentence. (A conjunction is a short word, such as and, or, or but, which is used to join two thoughts.)
The same thought could be expressed by combining dependent and independent clauses: “Because Mary takes music lessons (dependent), she plays the violin well (independent).” Combining a dependent and independent clause creates a complex sentence.
While a comma always follows a dependent clause which opens a sentence (as above), it is not mandated where the independent clause opens the sentence: “Mary plays the violin well (independent) because she takes music lessons (dependent).” A comma might be placed before “because,” but generally it is omitted if the sentence is relatively short, or the flow of the sentence would be interrupted by a comma. It depends on whether the dependent clause is restrictive or non-restrictive (remember them from the last time?); a restrictive dependent clause would not be set off by a comma but a non-restrictive one would be when it follows an independent clause.
There are some negative absolutes:
- Never use a comma to separate two independent clauses unless the comma is followed by a conjunction. “Bill went to the store, he bought a pen.” (Wrong unless a conjunction is inserted after the comma. As written, it is a run-on sentence.)
- Never use a comma to separate a subject and its predicate: “John who was late, ran all the way to school.” (Wrong; should be no comma.)
- Never use a comma to separate two predicates of the same subject: “Bill worked late, and drove home in the dark.” (Wrong.) “Worked” and “drove” are both predicates of the subject, “Bill.” The phrase “. . . and drove home in the dark” may look like a clause, but it is not because it has no subject. Note though, the mere addition of “he” changes the simple sentence to a compound sentence, and the comma is proper because “drove” now has its own subject: “Bill worked late, and he drove home in the dark.”