The difference between further and farther seems to confuse authorities on style and writing just as much as writers themselves. The CSE Manual, for example, suggests to use further as a verb and farther as an adjective or adverb, whereas the online tool Grammarly suggest using further for figurative and farther for physical distances. Why is it so difficult to choose between further and farther?
First of all, both words can be used as a verb, an adjective, or adverb. They can also have different meanings in some of these word categories, which complicates the matter even further (or farther?). To understand the distinction, let’s take a look at the usage of further and farther for each word category.
As a verb
When used as a verb, the right choice is always further.
She read a lot of books to further her general knowledge.
As an adjective
The adjective use is currently separating into two distinct uses: Farther is taking over the meaning of distance and further the meaning of addition.
the farther side of town (distance)
For further information, please contact costumer service. (addition)
As an adverb
As adverbs, both further and farther are used interchangeably.
I was too tired to walk any farther.
OR I was too tired to walk any further.
However, in two instances, the use of farther is no longer acceptable: when there is no notion of distance and when it is used as a sentence modifier. So, now we know that my choice for using further instead of farther in the sentence above was correct.
They can also have different meanings in some of these word categories, which complicates the matter even further. (no notion of distance)
Further, I would like to address the problem of funding. (sentence modifier)
When in doubt, go for further
Although there is a tendency to using farther for notions of distance and further in other cases, you cannot go wrong with further. So, for the lazy ones out there, just remember: When in doubt, go for further!
Sources:
The CSE Manual for Authors, Editors and Publishers, 8th Edition