Adverb: both words mean “more distance,” but only further can also mean, “additionally” (short for furthermore). Adjective: both words work as the comparative form of far, but only further can also mean “additional.” Verb: as a verb, further means “to advance” or “to progress;” farther cannot be used as a verb.

The widely accepted rule is to use farther when being literal and discussing a physical distance, as in “He went farther down the road.” Further is used when discussing a more symbolic distance or to discuss a degree or extent, as in “I wanted to discuss it further, but we didn’t have time.”

Both farther and further are correct. You might think they’re synonymous, but they have different meanings and uses in English. The standard quick answer is “ farther is for physical distance, while further is for figurative distance.”

For a time, speakers and writers did not differentiate between the two terms. However, a recent rule has established a clear separation between them. "Farther” refers to physical distance, while “further” refers to metaphorical distance, extension of time, or degree. "Further" can also mean "in addition to."

One reason farther and further are difficult to distinguish is that both mean something close to “beyond.” However, there is a big difference. Farther generally refers to physical distance, whereas further can extend to more-abstract concepts that are harder to measure.