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

First of all, if it is two clauses that can stand independently, then always use a comma. In the first example you give ("He read the book, and he really enjoyed the penultimate chapter.") The two clauses could in theory stand alone. However, I would say that in the second example ("They appear in groups, and are multiplying rapidly.") the comma is misused. The correct form would read, "They appear in groups and are multiplying rapidly." In the case that there were more than two verbs being used, a comma would then be necessary. For example, "They appear in group, are multiplying rapidly, and need more supplies." In this case the comma is used as the Oxford comma is used, to separate three or more things. I hope this is helpful in your writing endeavors.