Further vs Farther
Quick answer: Farther is for physical distance you could measure; further is for figurative distance, degree, or 'additional'. When in doubt, further is the safer choice — it covers both in modern usage.
Further
Adverb/adjective: to a greater degree; additional; also a verb (to further a cause).
“Let's discuss this further tomorrow.”
Farther
Adverb/adjective: at or to a greater physical distance.
“The trailhead is farther than it looks.”
How to remember it
FARther has FAR in it — real, physical distance.
Common mistake
'Nothing could be farther from the truth' — truth isn't a place; use further.