Commas in Complex Sentences

Complex sentences are sentences that have two clauses. They can have two independent clauses (each having a subject and predicate), or an independent clause and dependent clause (missing a subject or predicate). Whether a comma is used between them depends on the types and positions of the clauses.

An independent clause is a clause that can stand on its own as a sentence.

Example: I have a dog.

Example: It has a fluffy tail.

Often, two independent clauses can be joined with a conjunction, such as and, but, or, so, yet, or any other conjunction. A comma is placed right before the conjunction in this type of complex sentence.

Example: I have a dog, and it has a fluffy tail.

It is important to include a conjunction in the complex sentence above because without it, you will have committed a punctuation error often referred to as a comma splice.

Incorrect: I have a dog, it has a fluffy tail.

A dependent clause is a clause that cannot stand as a sentence in its own right, such as before I went home. When a complex sentence contains a dependent clause like this one, a comma is not used unless the dependent clause comes before the independent clause.

Example: Before I went home, I checked to make sure I had my keys.

If the positions of the clauses were reversed, with the independent clause first, there would be no comma in the sentence.

Example: I checked to make sure I had my keys before I went home.

Unnecessary Commas in Complex Sentences

Generally, if the dependent clause comes second in a complex sentence, a comma is not used.

Incorrect: I need to get a job, because I have run out of money.

Correct: I need to get a job because I have run out of money.

The second clause, because I have run out of money, is dependent; there is no need to use a comma between the two clauses.

Incorrect: It makes no sense to study French, if you are not going to France.

Correct: It makes no sense to study French if you are not going to France.

Correct: If you are not going to France, it makes no sense to study French.

In the last example, the dependent clause was used before the independent clause, so a comma was placed in this complex sentence.

Incorrect: I’ll turn the light off and sleep, when I can no longer keep my eyes open.

This sentence contains an unnecessary comma.

Correct: I’ll turn the light off and sleep when I can no longer keep my eyes open.

Exception: If the sentence is confusing or ambiguous without a comma, one may be used for purposes of clarity.

Example: Maggie didn’t call Alphonse because she was angry.

There are two ways to interpret this sentence. One possibility is that Maggie did call Alphonse, and her reason for calling was not that she was angry. The other possibility is that Maggie did not call Alphonse, and the reason she didn’t call is that she was angry. A comma removes the ambiguity.

Example: Maggie didn’t call Alphonse, because she was angry.