People frequently are unsure whether to put a comma before “and,” but how about using a comma before such words as because, when, where, and who?
Essential clause – Words that are an integral part of the main thought of a sentence form an essential clause, and you should not set off this clause with a comma. An essential clause will begin with such words as although, because, if, when, where, whether, or who (they are called subordinating conjunctions, and there are 23 in all, but you don’t need to memorize them in time for happy hour conversation).
Nonessential clause – When the information in the clause is not necessary to complete your main idea, insert a comma to separate it from the rest of the sentence.
Here is the distinction, as it applies to because:
A. He was allowed access because he has security clearance.
B. She cannot meet on Friday, because of schedule conflicts and deadlines.
Sentence A – The clause because he has security clearance is considered essential to the main thought because if you said only that “He was allowed access,” the reader would wonder why. With a significant question unanswered, the thought would be incomplete, or at least, the sentence would seem oddly clipped.
Sentence B – It is enough to say that she cannot meet on Friday. That’s all the reader needs to know. The reasons why provide extra explanation but are not necessary.
Here is another example:
A. Jim will meet you at the ski lodge where the other racers plan to gather.
B. Jim will meet you at the ski lodge, where the other racers plan to gather.
Without the comma, sentence A indicates that the clause where the other racers plan to gather is essential; that is, it means there are several ski lodges, and Jim plans to meet you at the one where other racers will gather, not at the lodges where racers will not be gathering.
But most ski resorts only have one lodge, so B illustrates the correct punctuation, because the clause where the other racers plan to gather is nonessential. If you were to write only that “I will meet you at the ski lodge,” the reader would know exactly where to meet you, because there is only one lodge. So if the words are not necessary, insert a comma to set them off.