Well, that depends on who you ask. Whereas hoard and horde have very different meanings, the definitions of “farther” and “further” are very much like one another. However, there are some critical differences that you may want to consider.
For starters, only “further” can be used as a verb. You can say that you want to further your education or further your career, but you can’t say that you want to “farther” your education. In this usage, “further” would relate to advancement, enhancement, and other similar notions.
That said, the usage of “farther” and “further” as adverbs can lead into some muddled gray territory. In the strictest sense, you could say that they are largely interchangeable, relating back to writing style (like using sentence fragments), but purists may argue that there is a difference.
In general, “farther” could refer to something that is greater in distance.
Toronto is farther from Vancouver than Calgary is.
Move the desk a few inches farther before setting it down.
By contrast, “further” could refer to something that is greater in magnitude or degree.
Obama could have gone further with his health care initiatives.
We can discuss this subject further at a future date.
For me, I tend to play it by ear. If it “sounds” right in the right context, then I’ll go with that word choice. This is similar to how I approach the usage of what’s to say, among other English phrases.
This can be tough for non-native speakers to understand (English is a quirky language). So, as before, the best advice I have for improving your writing skills (and grammar) is to read as much quality content as possible.