It's true, trying to explain something to someone and teaching it helps you to understand the subject inside and out. It also brings to light how much you may or may not actually know one the topic that you thought you might have.
And my hands are fairly decent a lot of the time. I used to practice on them and say that hands were pretty much the only thing I could draw well, and back in the day, it was true. But then I stopped drawing them for a time to concentrate on other things and it was as if I'd forgotten what I was doing, lol. So now I sometimes struggle with hands, and feet as well, but I make do.
Worse comes to worse, I can always draw something to cover the hand xD
As for anime fingernails, some artists draw them, some don't. It's kind of relative, just like with western art as well. More details add a sense of realism a good deal of the time though, taking away from the cartoon-y feel a lot of anime has.