Теорія:

Farther or Further?
Farther refers to length or distance.
It is the comparative form of the word far when referring to distance.
 
Further means to a greater degree, additional, or additionally.
It refers to time or amount. It is the comparative form of the word far when meaning much.

Correct: London is farther north than Juneau.
(Refers to distance).
 
Correct: This plan requires further study.
(Meaning additional study, refers to amount).
 
Correct: According to my timetable, we should be further along.
(Refers to time).
Older/Oldest or Elder/Eldest?
Elder and eldest can only be used to speak of persons, not animals or things.
Note that these two adjectives are now used only to talk about the members of the same family.
 
My eldest brother is in the Army.
Alice is my elder sister.