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.

Commas before "Because"

Summary: you rarely need a comma before "because," even when the word connects two independent clauses. The reason for this is that using a comma reduces the causal relationship between the clauses.

Writer's tip: don't use a comma before “because,” unless the first clause contains a negative, or the second clause is nonessential.

See also: Using Commas With “As”

Explanation:

While "because" often connects two independent clauses, and we are taught to place a comma before conjunctions connecting independent clauses, there is a rule that supersedes this one and tells us not to use a comma with "because" (in 99% of cases).

The rule is as follows:

If the clause is nonessential, use a comma before it, and conversely, if the clause is essential, don't use a comma.

E.g. She was Terrance's girlfriend, the same girl George had seen eating with Mark on Sunday. ("She" was identified as Terrance's girlfriend, the fact that George saw her eating with Mark on Sunday is nonessential.)
With an essential clause, the information is necessary to understand the sentence, so no comma should separate it from the clause which preceded it.

E.g. She was the one who had eaten with Mark on Sunday. (Without the clause, "who had eaten with him on Sunday," we wouldn't know who "she" is, so the information is critical to the meaning of the sentence.)

By its very nature, "because" makes the clause that follows it essential (due to the causal relationship that the word implies), and the comma should be omitted.

Here is an example:

He went to bed because he was tired.

Technically, "He went to bed" and "he was tired" are both independent clauses because they each have a subject and a verb, but when you join them with "because," it would be incorrect to use a comma.

Some more examples:

There was nothing to eat because the cupboard was bare.

He always ate alone because he didn't like talking with his mouth full, and he hated cold food.

When a comma before "because" is necessary:

1. He didn't run because he was afraid. (Then why did he run?)

He didn't run, because he was afraid.

2. He wasn't happy because they lost. (Then why was he happy?)

He wasn't happy, because they lost.

3. She didn't like him because he was tall. (Then why did she like him?)

She didn't like him, because he was tall.

Notice that in each case, the version of the sentence without the comma seems to say the opposite of what the one with the comma does. Moreover, the versions without commas don't even seem like complete sentences; there seems to be a missing clause in each. But also note the pattern: the first clause in each case used a negative--"didn't" or "wasn't."

Therefore, here's a good writer's tip: don't use a comma before "because" when joining two independent clauses, unless the first clause contains a negative.

Source

Commas before "Then"

A comma is sometimes required before the word "then."

The rule is fairly simple: "then" can mean "at the appointed time" or "in that case," and if you're talking about the latter, then you usually need a comma before the word. If you're talking about the former, you almost never need the comma. The exception to this is if you're using "then" as a conjunctive adverb to connect parts of a sentence. In that case, you need a comma before it.

For example:

We went to the beach then. ("Then" is the time that we went to the beach, no comma is needed.)

It's decided, then. ("Then" is used to mean "in that case," so a comma is needed.)

He opened the door, then stepped inside. ("Then" is being used as a conjunctive adverb, so you need a comma before it. Note, however, that if you use a real conjunction with the adverb you don't need the comma. E.g. He opened the door and then stepped inside.)

Source

When Semicolons Replace Commas

1. Semicolons are used to separate items in a list when the items already contain commas.

Normal List of items:

Inside the basket of fruit there was an apple, an orange, a banana, and a bundle of grapes.

List requiring semicolons:

Inside the basket of fruit there was a juicy, red apple; a shrunken, unripe orange; a half-eaten banana; and a bundle of overripe grapes.

Note: The last item has no commas, but a semicolon was used anyway. In mixed cases, I think, but have been unable to confirm, that you should use a semicolon if it makes the list easier to read. Just make sure you don't mix commas and semicolons within the same list. Be consistent.

 

2. Join independent clauses with a semicolon when one or both clauses contain a comma (Rozakis 156).

For example:

Wickham, a skinny kid with a perpetually hungry look, left his food, even though he hadn’t had enough; but George, who was full, couldn’t be blasted free from his plate—some people simply have stronger stomachs than others.

 

Sources

Rozakis, Laurie. English Grammar for the Utterly Confused. McGraw-Hill Companies, Inc., 2003.

http://www.cliffsnotes.com/WileyCDA/CliffsReviewTopic/Uses-of-the-Semicolon.topicArticleId-29011,articleId-28988.html