Earlier today, I had to pull out my trusted AP Stylebook. I had to explain why anyone was wrong in a sentence and couldn’t quite remember why it was wrong, I just knew it was.
The sentence was, “any one can participate.” I just knew it was supposed to be, “anyone can participate,” but I couldn’t remember why.
Luckily, the AP Stylebook always has the answer.
One word for an indefinite reference, two words when the emphasis is on singling out a member from a group.
Indefinite reference: Why would anyone want to do that? (This is the most common).
Singling out: Any one can vote. (The group is whoever is eligible to vote.)
Still confused? Check out this post by my favorite Grammar Girl on the topic.