Farther vs Further
Fartheradverb
to or at a greater distance in time or space
Furtheradverb
in addition or furthermore
The difference
Farther is for physical distance. Further is for degree or extent. He walked farther than us; let us discuss it further.
How to keep them apart
Farther contains far, which is a distance. Further is also a verb — to further a cause — which farther never is.