Even for native speakers, the English language is full of booby traps. That's why people are so hesitant to use whom instead of who, and why thinking about the differences between lay and lie is enough to give professional linguists a headache. One of the more common pitfalls is further vs. farther: Both words describe similar situations, and there's only one letter separating them. Though they're often used interchangeably, there is a difference between further and farther, and luckily for anyone who struggles with grammar, there's an easy trick to remember what it is.
Further and farther are both used in relation to progress, but the type of progress they describe differs. According to Quick and Dirty Tips, farther is reserved for physical distance, i.e. "the runner was farther down the track than his competitor," while further is used for figurative or metaphorical scenarios, such as "the senator was interrupted before she could go further in her speech."
The best way to remember this is to look at the first three letters of the words. Farther starts with far, a word that's associated with physical distance. This can remind you to use farther when describing things like car trips and walks, and save further for concepts like projects, movies, and dreams.
This distinction is clear enough, but things can get sticky when it's not totally obvious if a statement is dealing with physical or metaphorical distance. Take the sentence "the writer had gotten farther in her poem by the afternoon" as an example. If the progress being referred to is lines on a page, farther works just fine, but if the speaker is talking about the poem as a piece of art, further may be more more appropriate. In such instances, it's usually safest to default to further: Usage for farther is slightly stricter, and because further deals with situations that are already hard to define, you can get away with using it in more contexts. And if you still get them mixed up, don't let it bother you too much. Merriam-Webster notes that great writers have been using farther and further interchangeably for centuries.
[h/t Quick and Dirty Tips]