I am going to start this topic today and will keep coming back to it over the next weeks. The question is what to do with an element that begins a second sentence after an and or but (or, nor).
One of the basic reasons to have punctuation is to sort out the structure of the sentence.
Cardinal Rule No. 1: When two sentences are linked by a coordinate conjunction (and, but, or, nor), there is a comma — or some punctuation — before that conjunction — end of story. Though there is a notation that the comma MAY be left out when the two sentences are short, that is a MAY rule, not a “You have to” rule.
The element that comes at the beginning of the second clause often causes a problem. There are so very many variables, and there are three choices: no commas at all, commas around it, or one comma after it.
In the case of a parenthetical, there are commas around it. When there are commas around it, the comma before the conjunction has to change to a semicolon so that it stands out. Marking the division of the two sentences is the most important mark of punctuation in the sentence.
…We had hired him in May, but he simply didn’t work out.
…We had hired him in May; but, in our opinion, he simply didn’t work out.
…We had hired him in May; but, well, he simply didn’t work out.
More to come on this topic.