Do I Need to Use a Comma Before "As"?

In many sentences, if there is no comma before as, then as means “in the way that” or “while.” When you insert a comma before as, its meaning changes to “because.”

Many writers forget to add a comma before the word as when one is needed, or they add one when they shouldn’t.

These mistakes are especially common when as separates two independent clauses, as in the example below:

George cleaned the house as his wife had asked him to. George cleaned the house, as his wife had asked him to.

Both examples are grammatical, but the comma in the second sentence changes the meaning of the sentence because it changes what as means.

In the first sentence, with no comma, as means “in the way that.” In the second sentence, with a comma, as means “because.”

In some sentences, adding or omitting a comma could result in an illogical sentence.

For example, "George listened to the radio as he cleaned the house" is a perfectly fine sentence. But if we put a comma in there ("George listened to the radio, as he cleaned the house"), we get a sentence that, even though it's grammatical, makes no sense.