I found this in the “Merriam- Webster’s dictionary of English usage” (page 967) and immediately had to post it. I wouldn’t have imagined that I would ever find such a sentence in a dictionary….Don’t you just love the sound of the highlighted part? I do, but sometimes I wish there were more dictionary entries like this one!
mpurk 
I’d love this to be the explanation of when to use which tense! Actually, it is my personal rule. 😉 Unfortunately, there are many many exceptions I can’t seem to explain that easy. 🙂 Anyway, cool dictionary entry!