“What’s the difference?” It’s the question of a learner, a thinker, a person who’s comparing two not-quite-the-same concepts.

When I taught English in Asia I realized that motivated students have lots of these questions. I remember being asked, “What’s the difference between ‘affect’ and ‘effect’? Or between ‘than’ and ‘then’? Or ‘farther’ and ‘further’?” Most of the time I knew, but occasionally the question itself caused me, the teacher, to realize that I didn’t understand clearly.…