You guys are making my head hurt.

I heartily recommend this book, in whatever edition it's in now. I have the first, just for nostalgia's sake, and the 4th, which is dogeared, underlined and highlighted to heck and back. I also used it when I was teaching. My students loved it.

-- grammar curmudgeon 2 --

Edited to add that I recommend it because it has a large section in the back that deals with just these sorts of usage questions and gives very clear explanations. You don't have to be a J-student to use it.