Here’s the general rule of thumb…
Use the word “farther” when referring to physical distance and use the word “further” when referring to figurative distance.
Examples of the correct way to use “Farther”:
“Jane has to drive farther than John to get to the mall.”
Here we are talking about the physical distance Jane has to drive to get to the mall.
“I don’t want to run any farther”
Again, we are talking about the physical distance one is running.
Examples of the correct way to use “Further”:
“I’m not going to discuss this topic any further.”
The length of a discussion would be considered a figurative distance.
“Jane was worried that the further her mind travelled, the more scared she would get.”
Although “travel” is often considered a physical distance, a mind travels figurative distances so we would use “further” instead of “farther.”
Now, here’s the tricky part. What about those pesky sentences that don’t clearly state whether they are referring to a physical or figurative distance?
“How much further/farther did you get in your book?”
Is this sentence referring to the physical distance a person made in a book, as in the number of pages they read? Or, is it referring to the figurative distance, as in how far they are into the story?
In cases like these, it is advised that you use your best judgment. If you intend for it to mean the physical distance then use “farther,” but if you intend for it to mean the figurative distance, use “further.”
Lastly, what about “furthermore?”
Well, you can’t exactly say…
“Further, I wish to discuss the finer details in this article.” (THAT’S INCORRECT!)
I know that this may sound right, but “further” does not mean “in addition” or “moreover.” However, “furthermore” does. So, when looking to replace “in addition” or “moreover” use “furthermore.”
As in, “Furthermore, I suggest you stop reading this and start using ‘farther,’ ‘further,’ and ‘furthermore’ correctly.”