Farther and further are two words that seem very much alike and are commonly misused. While each has roughly the same meaning, they have very distinct usages.
Farther is used when you are discussing an actual distance. “I ran 100 yards farther than you.”
Further is used when you are trying to say “to a greater extent” or “more.” “He continued his lecture by further expanding his thesis.”
That’s it. Simple right? Well, when you can think about it, yes. When you use it on the spur of the moment, you just have to train yourself. I find it helpful to remember that farther = far and further does everything else. It is also helpful to remember that if you can replace the word with “more,” you should use further.