Here's something that you might find in a textbook:
Farther: only refers to distance. For example, "Neptune is farther from the sun than Venus."
Further: means 'additional' or 'subsequent'. For example, "Does anyone have any further questions?"
Many American speakers make this distinction. However, in everyday British English, it is acceptable to use 'further' in both contexts. Most British English speakers would say "Neptune is further from the sun than Venus." I presume that you're focusing on BrE, as I can see from your profile that all your teachers are from the UK. So if you want to make your life easy, you can probably forget about 'farther' - you'll never need to use it.
By the way, I never use the word 'farther' at all. (Except when talking about my Dad!)*
* A play on words that only works in non-rhotic accents.