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 (even good ones) forget to put a comma before the word as when one is needed (or they use a comma when they shouldn’t). Writers tend to make this mistake when forming sentences in which as separates two independent clauses.

Below is an example of such a sentence. The comma is omitted in the first example and is included in the second.

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

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

Both of these examples are grammatical. However, they do not mean the same thing, and so it is important to know how the use of a comma changes the meaning of the sentence.

Adding the comma did not change the meaning of any words in the sentence except for one: as. In sentences with the structure of our example, 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.”

So, the first sentence above, which contains no comma, can be rephrased as “George cleaned the house in the way that his wife had asked him to.”

The second sentence, which contains a comma, can be rephrased as “George cleaned the house because his wife had asked him to.”

Note that, for certain sentences, there is only one correct decision regarding the use of a comma based on the context.

George cleaned the house as he listened to the radio.

George cleaned the house, as he listened to the radio.

Putting a comma before as in this sentence is a mistake. George clearly cleaned the house while he listened to the radio, not because he was listening to the radio. The second sentence is still grammatical, but it isn’t logical.