farther / further
Physical distance versus degree, and the verb.
Farther is the safer word for physical distance: farther down the road. Further is the safer word for degree or addition: further delay, further study. Further is also the verb: further the work. Farther used for degree is common. Keep further for the verb.
Examples
Usefarther down the hall
Usefurther review
Usefurther the plan
Avoidfarther the plan