Comma before ‘because’ in English

by Jakub Marian

Tip: See my list of the Most Common Mistakes in English. It will teach you how to avoid mis­takes with com­mas, pre­pos­i­tions, ir­reg­u­lar verbs, and much more.

“Because” introduces a dependent clause that almost always contains essential information (the whole sentence would sound incomplete without it), so just like before “that” and similar conjunctions, we usually don’t use a comma, as in

correct I must leave now because my boss asked me to help him.
see below I must leave now, because my boss asked me to help him.

The latter example is grammatically correct but implies that the fact that your boss asked you to help him is not importantbut why mention it then? A good rule of thumb is: If you don’t feel the need to put the clause starting with “because” in parentheses, don’t use a comma.

There is one important class of exceptions, however. When the first clause is negative, not affirmative, it is often recommended to use a comma to avoid possible misreading. The Chicago Manual of Style gives the following example:

He didn’t run, because he was afraid.

This sentence can only be interpreted as “He didn’t run, and the reason was that he was afraid.” If we don’t use a comma, it can be misunderstood as, “The reason why he ran wasn’t that he was afraid”, as in

He didn’t run because he was afraid. He ran because he enjoys running.

If the meaning is clear even without a comma, you can omit it, but you should use a comma whenever the first reading can result in misunderstanding.

Note that if the order of the because-clause and the main clause is reversed, we always use a comma, just like for “if”:

correct Because he was afraid, he didn’t run.
wrong Because he was afraid he didn’t run.

This article was based on my guide to the most common mistakes in English, which explains many similar topics. Why don’t you check it out?