Unlike many rules on commas, the rule for when to use a comma before because is rather straightforward. Because is a subordinate conjunction – it connects a subordinate clause to an independent one. No comma is needed between them.
The clause beginning with because answers the question why? and is called a clause of purpose. The meaning here is clear – it tells us the purpose of the action of the independent clause. By definition, the clause starting with because is subordinate to the independent one. Because has the meaning of for the reason that.
I went to the store because I was hungry. ✔
I went to the store, because I was hungry. ✘
I slept in the car because my wife didn’t appreciate her gift. ✔
I slept in the car, because my wife didn’t appreciate her gift. ✘
Because as a Preposition
Because can also serve the role of a preposition, but only when it is followed by of. Because of is used to say that someone or something is the reason for something. The rule for commas is the same – no comma is needed before because of.
I had to leave because of his crankiness. ✔
I had to leave, because of his crankiness. ✘
Exceptions to the Rule
There are certain cases where the comma is not necessary, but it is preferable. You should put a comma before because when you wish to avoid ambiguity. The use of commas when the sentence structure is inverted is slightly different and will be explained below.
In sentences where the meaning will be clear only if you put a comma before because, do it. This mostly happens with sentences that start with a negative statement. The rule applies both to because and because of.
He didn’t leave because he was tired.
He didn’t leave, because he was tired.
Both sentences are grammatically correct, but they can be interpreted differently. The first example can mean that he did leave, but for reasons other than being tired. The meaning of the second example is clear – he did not leave, and the reason was that he was tired. See what a comma does?
Janet didn’t pass the test because of her level of knowledge.
Janet didn’t pass the test, because of her level of knowledge.
The first sentence can be interpreted as if Janet did pass the test, but her level of knowledge was not the crucial factor. In other words, Janet cheated. If your intended meaning is that Janet didn’t pass the test and her level of knowledge was the reason, put a comma before because of. The comma makes the causal relationship clear.
Inverted Sentence Structure
The use of commas in these cases is not directly related to because, but to the inverted structure of a sentence. When you flip the independent and dependent clauses, use a comma at the end of the dependent clause.
He bought her flowers because he likes her.
Because he likes her, he bought her flowers.
She was unhappy because of her injuries.
Because of her injuries, she was unhappy.
The Simple Response
The rule of thumb is not to use a comma before because. You should put a comma before because only in sentences where the meaning is ambiguous and you wish to clarify it.
She fell in love because he was handsome. ✔
She fell in love, because he was handsome. ✘