The right time to think about punctuation is after one has a good grasp of the basics of sentence structure. This takes the guesswork out of deciding where to place a comma, for example. English grammars and manuals will go on for pages about this particular punctuation mark, so let’s look at a few of the more common uses of the comma and the sentence structure behind them.
I am of the belief that next to the verb, the most important part of speech is the conjunction, at least from the practical concern of analyzing what we read and hear. Conjunctions are those small but powerful particles that group words together, and the punctuation we use (or do not use) with them can determine whether or not our reader will understand without too much trouble what we mean to say. The conjunctions and, but, or, and so are called coordinating conjunctions, and there is a good, firm punctuation rule to follow in using them: put a comma before a coordinating conjunction in joining two independent clauses. Take this sentence as an example of that rule: We were reluctant to sell, but we finally decided to put our home on the market. The conjunction but is combining two clauses (we were and we decided), and the fact that each could otherwise stand alone and say something complete in its own right, marks them as independent. The coordinating conjunction but connects these two independent clauses into one compound sentence, and so a comma should precede the conjunction.
Another common use of the comma is after a longish introductory adverbial phrase. In the sentence With the real estate market so good, we decided to put our home up for sale, the first seven words say something about the circumstances in which the subject we decided to do something. Any grammatical element (word, phrase, or clause) which points to the situation or condition or scene in which something else is taking place, works as an adverb, and when that adverbial element is fairly long, a comma is in order to help the reader separate more easily the circumstances from the action. If instead the introductory adverbial element is fairly short, no comma is felt necessary because the chances of confusion (or interfusion, really) are reduced: Last year we decided to put our home up for sale.
One more common use of the comma (and this one is a little more difficult) is with what is called a nonrestrictive element. In the sentence We decided to sell our home, a typical Chicago bungalow, the phrase a typical Chicago bungalow is meant simply to describe the home we sold—to describe it, not to define it or differentiate it from some other home we might own. The grammatical term nonrestrictive means descriptive, and another firm comma rule in English says that nonrestrictive elements are set off by commas. To understand better the difference between describing and defining, consider this sentence: We decided to sell our home in the country. Here the phrase in the country is meant not primarily to describe but to define the home we sold, the implication being that we had at least one other home which we were not selling, perhaps a home in the city. Such a defining phrase is called a restrictive element, and a complementary punctuation rule requires that nonrestrictive elements not be set off by commas. No comma, therefore, precedes the phrase in the country in this second example.
And one last use of the comma: nouns used in direct address are set off by a comma. If I wanted to leave a note for my friend to call me later, I would have to place a comma before (Call me this afternoon, Tom), after (Tom, call me this afternoon), or both before and after (Call me, Tom, this afternoon) his name because I am using that proper noun to directly address him, albeit only in print. It’s for the same reason we put a comma after the name in the opening of an informal email or letter (Dear Tom,), although a semicolon, not a comma, follows the opening in business or formal correspondence (Dear Tom:).
The important thing to remember is that commas cut, or separate, what is inessential or potentially confusing. With that in mind as we look at the grammatical structure of a sentence, we can usually determine much more confidently where and when to set a comma in its proper place.