I understand that a serial comma (the one used before and/or) is used only when we have three or more items to be separated using and/or.
What exactly is the rule that governs where to add a comma before an and/or in a sentence such as the following:
- S1: I went to John's house, and I watched a movie there.
- S2: I went to John's house and there I watched a movie.
- S3: Look around for any evidence that might reveal the identity of the intruder, and carefully place that evidence in a plastic bag, so it can be examined closely at headquarters.
I have seen S1 written both with and without comma. Which is correct; why?
What would be the right way to punctuate S2; why?
Should there be a comma before so in S3 or not; why/why not?