One of the good, reliable rules of English punctuation is that no comma is required after a main clause when a subordinate clause follows. So, for example, I’m going to go biking this weekend if it doesn’t rain. Clauses are made subordinate when they begin with a subordinating conjunction (when, since, if, after, although, because are some of the more common ones), and so in this example, there is no comma after weekend. If for stylistic reasons, however, we invert the order of the two clauses, a comma after the now initial subordinate clause would be required: If it doesn’t rain, I’m going to go biking this weekend.
That’s the rule. But a rule is a rule, not a diktat. What, for example, does this unsuspecting sentence mean: I didn’t call him because it was late. Is the fact that it was late the reason I didn’t call him, or is it precisely not the reason I didn’t call him? Do I mean to say because it was late, I didn’t call him, or something like I didn’t call him because it was late; I didn’t call him because I didn’t want to talk to him. The subordinating conjunction because will often produce this ambiguity when the main clause is negative, and it is good practice to redesign such sentences to preclude the possibility of misunderstanding.
The revision just given is one way to revise such a statement: begin the sentence with the because-clause and follow it with a comma. This takes care of the problem because the negative (the not in the contraction didn’t) is now contained within its own clause; we can’t feel its force if we haven’t come in contact with it yet as we read. But now we run into another problem. Because it was late, I didn’t call him sounds a little pretentious, a bit overly proper or exact. We would need to see the larger context in which the sentence appears to be sure, but we can revise out that apparent problem of diction by substituting since, another subordinating conjunction of cause, for the word because: Since it was late, I didn’t call him. We’re now closer to our natural voice, and pending any reason for a more formal structure, that should be our choice.
One further caution is in order, though. It is not uncommon to see an initial main clause conclude with a comma, even when positive and followed by a because-clause. Take, for example, this sentence: I’m going to go biking this weekend, because if I don’t take advantage of this last beautiful weekend of the year, I’ll be sorry all winter long. The comma after weekend serves a rhetorical, not a logical, purpose. The 14 words after the main clause create too large an element to hang unsupported, and the comma is necessary to section off the element for the reader’s easier comprehension.
This difference between rhetorical and logical punctuation often accounts for our frustration in deciding whether there should or should not be a comma somewhere. The best advice is to decide first on logical grounds, and then override that decision if the situation compels and the meaning is not impaired. Truth first is the rule. Which is why rules are merely rules and not authoritarian decrees.