It depends on whether you’re moving or staying put.
Farther refers to moving in the literal sense.
The farther we traveled, the more tired we became.
Further, on the other hand, is used to describe some sort of change, often a gain or improvement in a function or skill.
The students have progressed further than expected.
It’s possible to use both in a sentence.
She walked farther to further her knowledge of the native plants.
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