Using Commas With "As"

Summary: When using “as” to mean “at the same time” the clause becomes essential and shouldn’t be preceded by a comma, but when using “as” to mean “because,” the clause is nonessential and requires a comma (Strunk and White 5). “As” is the nonessential form of “because.”

Writer’s tip: With “as” the reason is a nonessential clause but the time is an essential clause.

See also:Using Commas With “Because”

Rule 1: When you’re using “as” to mean “at the same time,” omit the comma.

For example:

They left the theater as the movie started to play. (Here “they” left the theater at the same time as the movie is starting to play. Adding a comma before “as” would change the meaning).

They left the theater, as the movie started to play. (Did “they” leave the theater “because” the movie started to play? The comma before the word implies a causal relationship.)

Rule 2: If you’re using “as” to mean “because,” then use a comma.

For example:

Fred left the theater, as he didn’t like the movie.

Fred left the theater as he didn’t like the movie. (Is Fred leaving the theater while not liking the movie? The meaning is unclear.)

Pricilla liked the play, as it obviously had an excellent costume designer.

Pricilla liked the play as it obviously had an excellent costume designer. (The second clause is clearly an aside—nonessential—and requires a comma.)

George left as he saw his ex arrive. (George is leaving at the same time as he sees his ex arrive.)

George left, as he saw his ex arrive. (George is leaving “because” he saw his ex arrive.)

Rule 3: When using “as” as an adverb, treat it as you would another adverb, such as “then.”

When using “as” as an adverb it can mean “to the same degree” or “in the idea, character, or condition of.” In this case, it usually requires a comma before it, but follow the rule of essential and nonessential clauses.

For example:

Fred went to the country, as was his custom during the holidays.

Fred loved Christmas, as did his whole family.

In both cases the second clause is nonessential and requires a comma before it.

Sources:

http://www.englishforums.com/English/UsageOfCommaAs/bxmvq/post.htm

Strunk, William, and E.B. White. The Elements of Style. Fourth Edition. Massachusetts: Allyn & Bacon, 2000. Print.