Cool! Thanks!
John and I/John and me is easy for me to figure out if I’m writing or typing, but if I’m speaking I can’t really figure it out in real time.
Start thinking of it like this:
We need to make refunds to ((whoever overpaid) the subscription fee).
The preposition “to” needs to have an object, right? Well, it’s not just the following word; it’s the entire following clause. It’s just unfortunate that the word “whoever” has to come next. But ultimately, the verb “overpaid” needs to have a subject. That’s the purpose for “whoever” in that sentence. It’s there to be a subject of a verb, not an object of a preposition.
Compare it to “We drove to me/my house.” Which word is correct here? Clearly, it’s “my”. Why? Because the preposition “to” takes the whole clause as its object, not just the following word.
I can’t get punctuation and quotations correct. I don’t understand why Americans put the sentence-enders inside the quotes. Consider:
Is Yoda’s best quote “There is no try?”
Is Yoda’s best quote “There is no try.”?
Is Yoda’s best quote “There is no try”?
I vote for the middle one, unless it’s not a full sentence. Then you style it as #3. And btw, I fully support the gratuitous use of the interrobang:
How can you not think that Yoda’s best quote is “There is no try.”?!
Theirs a difference between you sing the wright won wrong, and knot no wing witch means what.
I’m generally pretty good with grammar rules (although my fiancee and I had an amusing grammar disagreement the other day*), but I can’t get the lay/lie stuff right, ever. And even if I weren’t too lazy to think about it, I don’t actually know what’s correct when. If someone has a simple system for remembering that, I’d appreciate you passing it on!
*The argument was about whether the following sentence can be read so that the blank could be correctly filled by either “think” OR “thinks” (as opposed to only one being correct) depending on your interpretation of the modification at play: “[That suggestion is moronic] because there remains a small group of people who ______ that lying to authorities to resolve an inconvenience carries a certain loss of integrity.”
I still can’t figure out how to end parenthetical statements with emoticons.
I notice nobody has mentioned would of (would’ve) yet.
This is so going to rock your already frustrated world.
Try thinking of it as a mass noun.
I dropped the rice, but none’s on the floor.
I thought you bought more milk…there’s none in the fridge.
None of them is coming to my party. 
This is a great way to think about it. Thanks!
Merriam-Webster and a bunch of stuff I just googled say that it can take a singular or a plural verb depending on the object of the preposition. It can be though of as both “not one” and “not any.”