Use of commas with seemingly dependent clauses

Question

I'd like to know whether or not the following examples are correct in relation to comma usage with independent/dependent clauses.

Every time I come across examples to use commas in separating independent clauses with a comma, it follows a pattern such as "He read the book, and he really enjoyed the penultimate chapter." I don't have trouble with this one.

Then I see examples, in published works, such as "They appear in groups, and are multiplying rapidly." Is leaving out the "they" between "and" and "are" just for the sake of reading with a better flow or something? Should it have just left out the comma?

"He was ready to lead everyone to safety, and victory against X". Without the comma, would it be to suggest "safety against X" as well? Hence, is it why a comma would be necessary in this case? Is it then treated as a "list", so to say?

So, in the cases of "They appear in groups, and are multiplying rapidly" and "He was ready to lead everyone to safety, and victory against X", are they both okay to use? Or perhaps the former is simply not, but the latter is necessary in such context?

Thanks for the help! I really appreciate it! (Racking my brain with commas - trying to edit a huge piece of work).

Answer 1

Is leaving out the "they" between "and" and "are" just for the sake of reading with a better flow or something?

Yes, though it was probably not a fully-conscious decision; people will "just" write or speak like that naturally. Deciding not to omit the they could come across as adding an undue emphasis.

Should it have just left out the comma?

Maybe.

For one thing the rule about commas and coordinating conjunctions (comma before the conjunction if they are independent clauses, not with dependent) is not one with universal agreement, especially when only the omission of a subject bars full independence.

For another, we could consider the second clause to be parenthetical. If intended to be considered as an additional thought, the comma becomes necessary.

Finally, there's just the rhetorical benefit of the pause. This is the reason you'll less often find in guides on grammar, but not to be underestimated. I don't know the context, but "…and are multiplying rapidly" sounds quite dramatic. I suspect the rhetorical benefit of the pause over-ruled other grammatical considerations for this writer.

"He was ready to lead everyone to safety, and victory against X". Without the comma, would it be to suggest "safety against X" as well? Hence, is it why a comma would be necessary in this case? Is it then treated as a "list", so to say?

Yes. It's not a matter of and as a conjunction between clauses, but between noun phrases in a list.

The object is "safety, and victory against X". Famously, one may or may not choose to use a comma before the final and in a list of three or more elements (the "Oxford comma" controversy). While there are arguments for and against such use as the norm, there are cases both where it can remove ambiguity and where it can cause it. This can also be the case where there are just two elements, and this is an example. While it's not normal to use such a comma when there are just two elements as a matter of course, it can still help remove an ambiguity, as here.