"Something, and since then, something else." - can "and" be in or out of the subclause constituents delimited by commas?

Question

Elsewhere someone asked a question about the following sentence:

[I] - I ate the pie, and since then, I have had a stomach ache.

Please consider the following variation (comma placement):

[II] - I ate the pie and, since then, I have had a stomach ache.

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1. In example I, what is the rationale for having a comma before the "and"?
2. Does example II make sense for the native speaker, and, if so, does the meaning differ from I?
3. Does the grammatical function of what's enclosed with the commas vary between I and II; is what's left, when we remove both the commas and what's inside, a consideration in that respect?

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I have taken a look at the CGEL. First I identified that you can have "since" alone with the meaning of "since then" for instance "...ever since"; but this is in the context of the realisation of terminal-point duration elements with adjuncts to clauses (chapter 8, p.708). Chapter 20 deals with Punctuation per se; I acknowledge the potential for variation. I'm trying to understand why you would(n't) include "and" in the subclausal constituents (p.1745) when using heavy-styled punctuation (p. 1727). Where I'm coming from, I'm thinking "if there was just 'and', and no 'since then', this would be more complete a sentence than the two parts marked with a single comma because is that even a sentence at all?" or... "why would you take a break in speech after the pie if you don't give a clue to introducing anything else; then shouldn't that pause be a period instead?". It's all very naive thinking, as I have no command whatsoever of formal grammar. I have asked questions accordingly.

Answer 1

[III] - I ate the pie, and since then I have had a stomach ache.

I think this is the most natural comma placement: it divides the two independent clauses, which is what commas do.

I don't know who wrote your Sentence I (SI). To me, putting the two commas here is unnatural. It represents to me an unnatural way of expression. It also means there are two pauses within five words and two pauses in a straightforward sentence of 13 words. I don't know why anyone would want to slow down that much.

The two commas in SII make sense, because they separate a unitary phrase (since then) from the rest. But although this two-comma version works much better than SI, it's not as eloquently simple and natural as SIII. This is because SII also introduces an unneeded double pause in a thirteen-word sentence that needs, at most, one pause. (Note that my use of commas to separate at most is similar to II's comma use.)

You could also dispense with the 'since then' and write the remaining with either no comma or one comma:

[SIV] I ate the pie and I have had a stomach ache.

To me this expresses both thoughts as one unit containing two facts. It does not really stress a causal or resultative relationship between the two actions/facts. Also a comma is just plain unnecessary because now the sentence is unencumbered by the 'since then', and it has only eleven words. I mean it is short and consists of two short independent clauses.

[SV] I ate the pie, and I have a stomach ache.

This goes back to the simple connection of the two independent clauses with a comma immediately after the conjunction, as in SIII. This is very frequently done, and at least helps the reader parse the sentence if not also slow down a tiny bit. (See how I used only one comma in that last sentence, and only to separate the two clauses? Oops, I just did it again.) SV probably does not present the two actions in terms of one unit of dual-facts like SIV. Perhaps because the pause also allows a millisecond for the reader to make some causal or resultative assumption between the two clauses--even though the comma by itself does not do that.

Answer 2

Commas are only required if the boundaries between clauses/phrases would be unclear without them, but commas sometimes do nothing more than mark natural pauses if one were to read the sentence aloud. This can change the emphasis of clauses/words and thus have subtle effects on the meaning of the sentence, even though the grammar or essential meaning of the sentence is often not affected.

I ate the pie, and since then, I have had a stomach ache.

I ate the pie and, since then, I have had a stomach ache.

The second sentence here has more of an emphasis on "since then". One reason for this emphasis would be that the speaker/writer is clarifying that he/she did not have a stomach ache until he/she ate the pie. While this conclusion is obvious from both sentences, the second sentence more implies that maybe the speaker/writer believes that his/her target didn't understand something the first time he/she said it, whereas the first sentence fits more in line with someone telling someone else the events for the first time.