I wouldn't say I'm an official grammarian, but I do like to diagram sentences, or did in high school. ;) Anyway, I found this article really interesting as I find myself correcting people in my head. Even though I still get confused by the rules/definitions/pronunciations. Here are some that hit home:
further - metaphorical distance or time
farther - physical distance
*I've been wondering about this lately.
supposably - conceivably
supposedly - "it's assumed"
*I hear people mis-speak this a lot. I don't really think they know that they're getting it wrong. Just like singing the wrong words to a song.
who - a subject
whom - an object
*That's always a favorite even though I rarely use them appropriately.
I always enjoy a little grammar knowledge. Nerd alert!!