How would you teach a confusing pair of words? As an example further and farther.

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It depends on the specific pair. Basically, you need something for your memory to latch on to. In the case of farther vs further, one is mainly used for comparing physical distances, while the other mostly refers to abstract progress. An easy way to differentiate them is that farther has 'far' in it and far is a descriptor of a physical distance.

"I swam out farther than was wise." "He jumped farther than his opponent."

"I made further progress on my goals." "He dreamed of further opportunities like this one."