The humble comma seems to baffle many. Space doesn’t permit full discussion of the subject (you may be relieved to hear), but here are some pointers.
If you’ve ever read a contract or a will drafted by an English solicitor, you’ll have noticed the complete lack of commas (omitted because they can be the source of ambiguity or error, as the cases attest*). Read that English document aloud, and you’ll be left breathless: no pauses.
The basic rule is that a comma should be inserted wherever there is a natural pause for the reader. Don’t overdo it, though: too many commas make things choppy. Lawyerly writing definitely suffers from excessive use of commas.
Use a comma to separate independent clauses joined by a conjunction (and, but, for, or, nor, so, yet): The plaintiff was severely injured in the accident, but the limitation period has now expired.
Use a comma to set off information that is useful but non-essential or in some way extra (it could be removed and the sentence would still make sense): The comma is, of course, a useful thing or The plaintiff, who has suffered multiple injuries, is unable to perform basic tasks.
Separate items in a list with a comma (or a semi-colon, if individual items within the list contain internal commas).
But, you ask, is there a comma (sometimes called a ‘serial’ or ‘Oxford’ comma) before the ‘and’ (or ‘or’) preceding the final item in the list? In the USA, generally; in the UK, generally not. Take your pick, but I tend (as in most things) to go with the Brits. That final comma looks fussy and a bit cluttered.
Sometimes it’s useful, though. This sentence could use a second comma, methinks: This book is dedicated to my parents, Margaret Thatcher and God. Ambiguities could also arise where a list is longer and more complicated.
The comma defines and limits. I asked my brother Ed for help and I asked my brother, Ed, for help mean different things. The first means I have brothers other than Ed; the second, only one brother, and he’s called Ed.
Commas are also used to separate multiple adjectives describing a noun: She drove a little, red Corvette. The comma drops out, however, if the final adjective is an integral part of the noun that follows: The birdwatcher spotted a beautiful bald eagle (unless it was lovely and had no feathers on its head).
Don’t separate what should not be separated – like subject and verb, or verb and object/complement. This is wrong: The thing is, that … [worse: adding another is before that]. And so is this: Many Somalis have termed the 2015 election, historic (from qz.com, 25 October 2015; the comma must go).
The comma does connect, but beware the run-on sentence, which (incorrectly) splices two independent sentences with a comma. Don’t do this: I am sending you this article, I thought you would find it interesting. Instead of that comma, you need a semi-colon, a period or a word like ‘because’.
This is correct: He said, “I disagree”, and then went on to add, “but I won’t argue the point with you.” Use a comma before quoted speech (or a colon, although that wouldn’t work in the example just given because of the structure of the remaining part of the sentence). Put punctuation outside the quotation marks unless it’s actually part of the quotation (the final period is, the comma after ‘disagree’ may or may not be). In the USA, it’s usual to put all punctuation inside the quotation marks, but this is not desirable.
Next: colons and semi-colons
*Some expensive comma cases: