These two words create a real pip of a problem for all English speaking peoples. I’ve even heard highly educated people get it wrong. They are both adverbs and have similar meanings, and this similarity causes the problem.
Further is distance by degree (Do give me further examples). Farther is distance by measurement (My horse runs farther than yours).
If the distance is not an actual distance in inches, yards, or miles, then the word is further. If the distance is measurable, then it is farther.
Nonetheless, the speaker/writer must consider what he is communicating when deciding which word is appropriate. Sometimes this takes a concerted conscious thought. On the Webster's Dictionary website, someone asked in the comments section if the following sentence correctly used the word further: “This is further to our memorandum yesterday.” I had to think about it a moment myself, but the word is used correctly. The problem is in the sentence itself. The subject of the sentence is weak. To what is the writer referring by her subject, “This is”? The overall meaning of the sentence is obscured. Let’s rewrite it, supplying our own idea of what “That is” refers to.
This last minute appendage is further to yesterday’s memorandum.
I don’t like this sentence, and if I were the writer, I would put it differently. Nonetheless, clarifying the sentence helps us know which word to use and also transfers meaning to the reader. Always remember: You are not writing for yourself but for your reader. Write toward your reader’s understanding.
Furthermore, don’t be confused by creative uses of these words. For example, “…my ponies are tired, and I have further to go…” from The Return of the Native by Thomas Hardy. In this passage, a character is speaking. Novel and movie characters do not always speak English correctly, so make sure to check your source before absorbing another’s bad behavior.
Synonyms for Further: added, another, else, fresh, additional, more, otherSynonyms for Farther: added, another, else, fresh, additional, more, other