Knowing when to use a comma with the word because is something that confuses even the most knowledgable editor. (Myself included.) Once you see a few examples, though, hopefully this will become more clear.
Chicago Manual of Style says the following: “A dependent clause that follows a main clause should not be preceded by a comma if it is restrictive, that is, essential to the meaning of the main clause.” [16th edition 6.31]
This means that you do not need a comma when the second part of the sentence is necessary in order for the first to make sense. Here are a few examples:
He wasn’t running because he was afraid; he was running because he was late.
I was only allowed into the building because I had a key.
In the above sentences, the second half of each sentence is essential to the meaning of the whole. He was late, and this is why he was running, not because he was afraid. I had a key, which is why I was allowed to enter the building.
Now, let’s take a look at some sentences where a comma is needed. This is where it gets confusing.
He didn’t run, because he was afraid to move.
At first glance, this looks almost identical to the sentence above. But consider: If you don’t have a comma, you get: He didn’t run because he was afraid. What’s wrong with that, you ask? The problem is, this makes it sound as though there’s a follow-up sentence coming, as though the reason he didn’t run was not because he was afraid but because of some other reason. However, the reason he didn’t run is because he was afraid; he didn’t run because he just didn’t want to.
However, because there’s a comma, you get a full sentence. He didn’t run. And this is why: Because he was afraid to move.
Essentially, you could say that all the reader really needs to know is that he didn’t run. The reason is not essential to the sentence and doesn’t change the fact that he did or didn’t run. In the above example with the building key, this information is essential, which is why no comma was used. You were allowed into the building, but it was only because you had a key. That’s important information, thus no comma was added.
Here’s another example:
I am not going to the store today, because I already have everything I need to make dinner.
You need a comma in this sentence, because otherwise the meaning is ambiguous. Is the reason I’m not going the store because I already have everything or for some other reason? Also, consider that the most important part of the sentence is: I am not going to the store today. The reason is extra information and not essential to completing the sentence.
The issue of when to use a comma before because is tricky, but it’s important, because using or not using a comma can sometimes entirely change the meaning of your sentence!
For more examples of because commas, see these helpful articles: