Language is kind of a hobby of mine.
It's not a hobby I give a lot of time to, mind you, but I do find so many things about languages fascinating. How they evolve, how they work, what makes this word better than that one, and so on-- it's all interesting. Here's a fun one for you: When I was learning Scots Gaelic, I learnt the phrase "Is math sin", which is pronounced "Ish ma shin", which sounds like "Smashing", which in Scotland is slang for "Good", which just happens to be what "Is math sin" means. Cool, eh?
As it happens, English is the language I have the most fun with, being as it is my native language and all. I've never had to work hard at English, which I'm pretty sure annoys other people to no end, but it's easy for me. (Hey, I have to put hours and hours into maths & sciences. Leave me alone.)
And therefore, it annoys me that I have a hard time with "farther" and "further". This is not an uncommon struggle; you will find this pairing on most lists of frequently confused words. But, I'm not used to confusing words. Give me "affect" and "effect" or "lie" and "lay" any day, and I've got you covered. And even more annoying is this: I know the difference between "farther" (refers to literal distance) and "further" (refers to figurative distance or degree) perfectly well; it's when applying them in everyday speech & writing that I have issues.
So here is where the real problem lies: I've never had to work at English, so I've no idea how to imprint all this knowledge onto my brain in order to make it habit. Sigh.