A correspondent writes to ask if there is a rule for the use of commas before end-placed optional adverbials.
The 'optional' is important. If an adverbial is obligatorily required to complete the sense of the verb, then there can't be a comma. An adverbial is obligatory after put, for instance, so it has to be:
I put the book on the table.
I put the book, on the table.
Similarly, if there is a clear semantic contrast involved, the comma is needed to make a distinction:
He walked naturally. = He walked in a natural manner.
He walked, naturally. = Of course he walked.
With optional adverbials, it would be unusual to see commas before single adverbs, adverb phrases, and short adverb clauses:
I stopped reading the book immediately.
I stopped reading the book at three o'clock.
I stopped reading the book when the clock struck three.
Inserting a comma in such cases would be to add a dramatic reading which would have to be justified by the context, such as - in this example - conveying an 'afterthought' impression:
I stopped reading the book, immediately.
= I stopped reading the book - immediately.
With clauses, because they are semantically and grammatically more self-contained, there is a greater likelihood of a comma intervening. However, this is only likely to happen if the adjacent clauses are relatively lengthy (motivating a pause and thus a comma) or if there is no close semantic link between adverbial and verb. The following pair of sentences illustrate the first issue:
Most people took up new jobs after the war was over.
Thousands of people of all ranks and ages took up new jobs in a wide range of professions, after the peace negotiations had been brought to a satisfactory conclusion.
The comma is not obligatory, but inserting one certainly helps the reading process, whether internally or reading aloud, and it is thus widely encountered in such contexts.
In that pair of sentences, there is no strong semantic link between the verb phrase took up and the adverbial. By contrast, the earlier sentence I stopped reading the book when the clock struck three does have a strong link. The temporal meanings of stopped and when form a strong semantic bond and make a comma unlikely.
The main places where there is comma uncertainty arise when there is a clash between these two criteria. A lengthy construction would motivate a comma, whereas a strong semantic link would not. This sentence illustrates:
I stopped reading the book about how to carry out an analysis of commas in a wide range of languages(,) when I realized that it wasn't going to reach any satisfactory conclusion.
Style guides vary in their advice, in such cases. Most newspapers underpunctuate.
Writers with a strong sense of auditory style are much more likely to use commas, to point the way they want their sentences to be heard. I believe this is a major feature of my own style. Indeed, it can be seen just now (in my to point clause) and a few sentences ago (where my whereas clause was separated by a comma). Grammatically the commas are unnecessary, in these cases, but they represent the way I want the sentences to be internally heard. The issue becomes a matter of aesthetics, now, and so not everyone will like it. Indeed, a few weeks ago I got a ferocious email from someone complaining about the overuse of commas in my By Hook or By Crook. He found four in one short sentence, he said. Me, overuse commas, in a short sentence? Never, never, never, never, never.