Here’s a bit about commas. It’s actually difficult to express grammatical rules systematically and in your own words, so I’ve likely made some mistakes along the way. What follows is less “here are the perfect, utterly true commanist ideals that I strive to embody” and more “here’s what I think that I’m doing when I write.”
Still. I know a lot of it’s correct. And I know a lot of other people’s commas are just… wrong. So here goes. We begin where we should always begin — in the middle.
I. Appositive nouns in the middle of a sentence.
A pair of appositive commas set off a noun or noun phrase that in effect renames a thing you’ve just finished naming. The second naming isn’t the grammatical noun in the sentence. Typically the second naming is just an elegant way to give more information to the reader without resorting to a another standalone declarative sentence:
My husband, the love of my life, is an aerospace engineer.
The proper inference to draw from them is that the one thing is the other thing, and vice versa. If you don’t mean to assert that, then don’t use appositive commas. In particular don’t use them like this:
Johns Hopkins history professor, Angus Burgin, will speak tomorrow.
I know for a fact that Burgin isn’t all alone at my alma mater. If he were, we should put the word “The” at the start of the sentence, while leaving the commas in place. In the real world, both commas should be deleted. “Johns Hopkins history professor” is not functioning as a noun phrase here, but as a modifier of title, much like the word “President” functions in the sentence “President Barack Obama will speak about gun control.”
Academic titles tend toward the long and highfalutin, particularly when we speak of endowed chairs, but in this respect they should still put their pants on one leg at a time just like the rest of us. As parts of speech, I prefer to treat them as modifiers of title. This then is correct:
Harry C. Black Professor of History Philip Morgan has written…
While to my ear this is technically correct but a bit pretentious:
The Harry C. Black Professor of History, Philip Morgan, has written…
It makes the title seem more or greater than the man who holds it. If for some reason you wish to place emphasis on the title, you may, and this is how you should do it. But generally you probably shouldn’t. It’s not the title who has written anything. It’s the man.
II. Appositive commas at the beginning.
We can also begin a sentence with an appositive — that is, with the second, descriptive naming, rather than with the naming that is serving as the noun part of the sentence:
An intellectual historian, Angus Burgin has written a fascinating book about F.A. Hayek.
We should exercise discretion in which thing we would prefer to function as the noun and which as the appositive. We signal that deliberate choice with a combination of word order and commas. That’s what they do. That’s how they’re supposed to work. It thus expresses the same idea as the just-quoted sentence if we write:
Angus Burgin, an intellectual historian, has written a fascinating book about F.A. Hayek.
…albeit with a slightly different emphasis. But the following would be both different and grammatically wrong:
Angus Burgin, an intellectual historian has written a fascinating book about F.A. Hayek.
…because the comma makes it quite unclear what function “Angus Burgin” performs in the sentence.
III. From appositive to introductory.
Note that when we move from general to specific, and when we’re not asserting a one-to-one identity, we should not use any commas at all:
American chess grandmaster Bobby Fischer won the world championship in 1972.
“American chess grandmaster” is an adjective, in effect, much like “Johns Hopkins history professor” has been just previously. It describes Fischer, but also a more general group of players. No commas should be used. And yet keep in mind:
The greatest American chess player of all time, Bobby Fischer, won the world championship in 1972.
No other Americans have ever been as strong as Fischer. The identity is exact — and appositive — and we use commas to say so.
But now consider:
In 1972, chess grandmaster Bobby Fischer won the world championship.
That’s an introductory element. Often, introductory elements are used to express time or circumstances. When the time or circumstances are specific or of long description, a comma is typically in order. Brief or very familiar introductory elements don’t always need commas. But unusual or lengthy ones generally do. Much of this is a matter of judgment.
But there are limits. “But” and “And” are so familiar as introductory elements that they should never take commas at all unless they constitute the first of two or more introductory elements in a complex set of them. For many other introductory elements, it’s okay to let your ear be the guide.
However, “however” is a special case. It has two possible functions at the start of a sentence. One is to indicate a qualification that applies to the sentence just before it. In that case you use a comma. However, however can also mean “in whatever way.” If that’s what you mean, do not use a comma. However you use the word, be sure the comma is correct, and remember that what distinguishes the two forms of “however” is the comma.
The comma of introductory material is absolutely necessary when you use introductory material that might otherwise throw your readers off. “And” and “But” are at the easy end of the continuum. But, and I will insist upon it, this surely is near the other end:
If you absolutely must write sentences like these, use a comma.
You’ll note a lot of additional clunk if you imagine the sentence without the comma.
Of course, there’s plenty of clunk there in the first place. It’s almost as if the writer began with one sentence, got lost in the middle, and finished up by writing a different sentence entirely.
The comma for introductory material forestalls any such untoward implications. Yet having to rely on it may itself be a warning sign. Too many merely introductory words before a comma will make for confusion; too many sentences that begin with introductory matter will make for ponderous reading. If either occurs, consider a rewrite for elegance.