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2. The second most troublesome use of the comma comes when a phrase within a sentence has to be separated from the rest of the sentence. To many students, the distinction seems totally arbitrary. It is not. Consider this sentence:
My grandmother who smokes pot is eighty.
Would you or wouldn't you put commas around "who smokes pot"? The answer is that you can do either; it depends entirely on what you mean to say. That is the important reason for going to all this trouble. Including the commas changes the meaning. If you don't know that, it's as if you were writing in a foreign language you do not understand. Imagine the possibilities for disaster!
Without commas, this sentence says that of your two grandmothers it is the pot-smoking one who is eighty. (The bourbondrinking one, presumably, is ninety-two.) Without commas, the phrase, as we say, is restrictive. It is there to indicate specifically which grandmother you are talking about.
With commas, "My grandmother, who smokes pot, is eighty" implies that you have only one grandmother, who is eighty and who (by the way) smokes pot. The commas separate out extra material that is added to the sentence but is not necessarily crucial to the main point, which is your grandmother's age. You could say "My grandmother is eighty," and the basics of the sentence would remain. You would, however, have just killed off your ninety-two-year-old bourbon-drinking grandmother. Way to go!
Another example of this same principle occurs when you are citing an author's books. Robert Frost poem "Stopping by Woods on a Snowy Evening" is a well-loved poem. Writing it this way, without commas around the name of the poem, specifies which of Frost's many poems you are talking about. It is restrictive. If you put in commas and write " Robert Frost poem, 'Stopping by Woods . . .'" you are saying that Robert Frost wrote only one poem, which (by the way) is called "Stopping by Woods . . . ." The difference between "Jane's husband Ted" and "Jill's husband, Fred," is that Jane is a bigamist. See?
The same principle applies elsewhere. "He stopped in Fairfax, Virginia, by mistake." Always put commas around the state or country so designated. This is because you are saying that Fairfax is (by the way) in Virginia. It is an extra bit of information like the year in the date July 4, 1776, when our country declared independence. I find that saying "(by the way)" to myself helps me to determine whether any particular phrase ought to or ought not to have commas around it.
For those who relish the finer distinctions, this is also the reason for sometimes using "that" and at other times using "which." You use "which" when the information in the phrase is incidental to the main idea, and you put a comma before the "which." You use "that" when the information is important and restricts the meaning of the main thought; in this case no comma is used. When you use "which," you should be able to think "(by the way)."