Could you please explain when to use a comma before as and because?
Commas are used as grammatical markers to help convey meaning in sentences. This means we have to look at the function of because in the sentence. Remember the 8 parts of speech?
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Do not use a comma before because when it’s an adverb.
When because relates to a noun phrase, it’s an adverb. It’s an adverb because it relates to why an action happened. I was promoted because of my competence. Notice that there’s no verb after “because.”
When because relates to a clause (i.e. contains subject and verb), then it’s a conjunction. I was promoted because I am competent.
As an adverb, because is not preceded by a comma. Example: I was arrested because of my inebriation. The meaning is clear without a comma.
For a conjunction, sometimes use a comma before because
However, as a conjunction, because may or may not take a comma. Unfortunately, there is no clear rule. It depends on the logic of the sentence.
Here’s an example: I put my socks on because my feet were cold. In this sentence, there is a clear, logical connection between the main verb—put—and the predicate phrase my feet were cold. But what if the logic is less clear?
I drove home from the party because Kerry was drunk. In this sentence, the logic is not clear. Did Kerry’s inebriation force me to drive home from the party? Did I drive for no other reason than I was sober? Did I drive home because I didn’t want to be around a pickled Kerry? A rational person might assume that I was driving because Kerry was in no state to drive, and they would be quite correct, but as writers, we must not leave room for doubt if we can avoid it. In this case, a comma would help clarify. I drove home from the party, because Kerry was drunk. By using a comma, the emphasis is on the main clause, which means “I” as the subject. So the important part is that it’s me driving, not Kerry. This helps to clarify that Kerry’s inebriation was the motivating factor for me driving. It still doesn’t clarify whether or not I left her at the party.
So, to determine the usage of the comma with “because,” we must consider if there is a possible logic that is not intended, and use the comma to eliminate any confusion on the part of the reader. This is really the most challenging thing for writers. We usually know what we want to say, and we believe we have said it. But a good writer (or a good editor) will recognize when alternative meanings will arise in the minds of readers and craft sentences that are clear and meaningful.
Comma before as?
Commas before as can be more tricky. Like because, as can be used as a conjunction or as an adverb. (Notice how I used it as an adverb in the preceding sentence.) Again, when it is used as an adverb, you don’t use a comma. Notice the difference in the following two sentences.
I drove the car home as it was snowing.
I drove the car home, as it was snowing.
In the first sentence, as is used as a coordinating adverb. Unlike the use of because as an adverb, here the noun phrase contains a verb. Although the phrase “it was snowing” can stand as an independent sentence, in this case, it acts as a noun: it is a statement of a fact. Two things happened simultaneously. In the second sentence, as is used as a logical connector so it’s a conjunction. For some unknown reason, the snow motivated me to drive. Perhaps the alternative driver was not confident of driving in the snow. Perhaps Kerry was too drunk to drive in the snow.
In addition, as can be used to make a simile. (The clouds are as fluffy as cotton.) In this case both instances of as are adverbs. Obviously, a comma before as in this case would be wrong: The clouds are, as fluffy as cotton. But what about a longer, more convoluted sentence? I drove carefully through the snowflakes falling as softly as feathers. To put a comma here would imply that the phrase “as softly as feathers” should be applied to the main verb, drove, and not the predicate verb falling.
Personally, I like to use a comma before “as” when it is used as a logical connector; however, I learned while researching the answer to this questions, that there is no rule requiring this. If the sentence is simple and the logic is clear, then there is no absolute need for a comma before “as.”
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