“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
The latter example is grammatically correct but implies that the fact that your boss asked you to help him is not important—but 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:
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
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”: