The descriptions in a sentence may be longer than one word. You may have a subject-verb expression (which grammarians call a clause) or a verb form (in technical terms, a participle). No matter what they’re called, In English these longer descriptions follow one simple rule: If a description is essential to the meaning of the sentence, don’t put commas around it. If the description provides extra, nonessential information, set it off with commas.
Consider this situation:
In her quest to reform Larry’s government, Ella made this statement:
Taxes, which are a hardship for the people, are not acceptable.
Lou, who is a member of Larry’s Parliament, declared himself in complete agreement with Ella’s statement. However, his version had no commas:
Taxes which are a hardship for the people are not acceptable.
Do the commas really matter? Yes. They matter a lot. Here’s the deal. If the description which are a hardship for the people is set off from the rest of the sentence by commas, the description is extra — not essential to the meaning of the sentence. You can cross it out and the sentence still means the same thing.
If commas do not set off the description, however, the description is essential to the meaning of the sentence. It may not be removed without altering what you are saying. Can you now see the difference between Ella’s statement and Lou’s? Here’s the expanded version of each statement:
Ella’s expanded statement: The government should not impose taxes. We can run the government perfectly well by selling postage stamps to foreign tourists. I suggest a tasteful portrait of the royal bride (me) on a new stamp. No taxes — that’s the bottom line.
Because Ella’s original sentence includes commas, the description which are a hardship for the people is extra information. You can omit it from the sentence. Thus Ella is against all taxes.
Lou’s expanded statement: The government is against any taxes which are a hardship for the people. No one wants to place a burden on the working families of our great nation. However, a 90 percent income tax is not a hardship; it pays my salary. This particular tax is acceptable.
Lou’s proposal is much less extreme than Ella’s. Without commas the description is a necessary part of the sentence. It gives the reader essential information about the meaning of taxes. Lou opposes only some taxes — those he believes are a burden. He isn’t against all taxes. This description doesn’t simply add a reason, as Ella’s does. Instead, it identifies which taxes Lou opposes.
The pronouns which and that may help you decide whether or not you need commas. That generally introduces information that the sentence can’t do without — essential information that isn’t set off by commas. The pronoun which, on the other hand, often introduces nonessential information that may be surrounded by commas. Keep in mind, however, that these distinctions are not true 100 percent of the time. Sometimes which introduces a description that is essential and therefore needs no commas. On rare occasions, the pronoun that introduces nonessential material.
Check out these additional examples, with the description in italic:
The students who are planning a sit-in tomorrow want to be paid for doing homework.
Punctuation analysis: The description is not set off by commas, so you may not omit it.
What the sentence means: Some of the students — those planning a sit-in — want to be paid for doing homework. Not all the students want to be paid. The rest are perfectly content to do math problems for free.
The senators, planning to revolt, have given the television network exclusive rights to cover their rebellion.
Punctuation analysis: The commas indicate that the description is extra, nonessential information.
What the sentence means: All the senators are involved. They’re quite upset, and all have prepared sound bites and scheduled press conferences.
The word because generally introduces a reason. At the beginning of a sentence, the "because" statement acts as an introductory remark is always set off by a comma.
Because the tattoo was on sale, Lulu whipped out her credit card and rolled up her sleeve.
At the end of a sentence, the because statement is sometimes set off by commas, in which case it may be lifted out of the sentence without changing the meaning. Without commas, it's essential to the meaning. Take a look at these two statements:
With commas: Lulu didn’t get that tattoo, because it was in bad taste.
Meaning: No tattoos for Lulu! The because information is extra, explaining why Lulu passed on the design.
Without commas: Lulu didn’t get that tattoo because it was in bad taste.
Meaning: Lulu got the tattoo, but not because it was in bad taste. She got it for another reason (perhaps the sale). The fact that the tattoo grossed out everyone who saw it was just an extra added attraction to Lulu, who enjoys looking strange.