Explanation

"Farther" and "further" are often used interchangeably, but there is a subtle difference between them:

Farther: It primarily refers to physical distance or advancement in a physical sense. For example, "The store is farther away than I thought."

Further: It generally denotes a figurative, metaphorical, or abstract distance, advancement, or progress. For instance, "We need to conduct further research on this topic."