Farther is physical distance. Further is everything else: more in degree, or “in addition.” ACT English treats the swap as a diction error. Those items are in word choice on ACT English.
Distance you could measure
The test center is farther from school than I thought. Miles, blocks, steps: farther. If you cannot put a tape on it, use further. We need further review of the timing errors. Further also means “moreover”: Further, the comma has to close.
Between and among
The same question sometimes pairs between with two items and among with a group. That is a comparison-word choice, covered in comparison words. A lookalike spelling pair, accept and except, is in accept versus except. Idiom choices that are not simple swaps are in ACT idioms.
The bottom line
If you could measure it in feet, use farther. If you mean “more” or “also,” use further.
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