farther / further
One is distance you could measure. One is everything else.
-
Wrong: Let’s discuss it farther.
Right: Let’s discuss it further.
Not a distance. Further.
-
Wrong: The store is further than I thought.
Right: The store is farther than I thought.
Measurable distance. Farther has far in it.
The mnemonic
Farther is about how far—actual, physical, measurable distance.
Further is for everything abstract: further discussion, further notice, further along in the process. It’s also a verb—to further your career.
The honest caveat
In British usage further covers both, and plenty of careful American writers use it for distance too. Nobody will misunderstand you.
What does get noticed is the other direction: farther used for something non-physical. Farther discussion has no defence. If you only remember one thing, remember that farther needs a distance.
Now go measure something.