Further and farther are the comparatives of far, but they have some difference in meaning and usage.
As adverbs, both farther and further can imply distance in space or time:
- The road is blocked, we can’t go any further/farther.
- We watched their train moving gradually further/farther away.
- Liverpool is further/farther away than Manchester.
- Nothing could be further/farther from the truth..
When the degree or extent of something is implied, the use of further is preferred:
- They didn’t get any further with discussing the plan.
- The police decided to investigate further.
As an adjective, further, but not farther, can also be used, mainly with abstract nouns, to mean additional, extra, more, more advanced:
- Are there any further questions?
- Further supplies will soon be available.
- Please turn to the manager with further inquiries.
- The committee decided to take no further action.
The adjective farther is only used for referring to distance:
- The farther shore of the lake could hardly be seen in the mist.
- Grandfather lives at the farther end of the village.