The word עצם means "bone" or the "stuff" you're made of. It is the "substance" of something. It is the "self." אופן means manner/method/way of doing something, but I happen to like "cycle" and the word is even adapted to describe riding a biCYCLE:
רכב על אופניים
Every revolution of the wheels is a cycle and many revolutions accomplish getting from point A to point B. I feel our given exercise parallels the English like this:
The boy studies English in a self-based cycle.
I hope my buddy Danny doesn't get upset (I know, friend, we can't translate stuff this way.)
Thanks Ingeborg. I'm glad you're here. Somewhere in these exercises was something about growing loads of carrots but also vegetables באופן כללי. I did find "cyclist" as a potential meaning for אופן (pronounced the same way) in one of the dictionaries and ofahn is at least a close cousin word meaning "wheel," as you pointed out. I deduced that both nouns employ the method of "cycling" and, therefore, the act of turning/rotating/making revolutions repeatedly was likely the basis for choosing the word אופן to portray the method/manner/way/custom in question... It repeats, comes back around, over and over again. That's how an ofehn eventually became an ofehn... Repetition. Using ofehn as "cycle" seemed to fit ok with regard to the "crop cycling" in the agriculture exercise. Now, I didn't dare translate it this way (for fear of the beloved trolls burning down a heart), but I did parallel באופן כללי as "in a customary cycle," momentarily, before actually inputting "in general."
Think I've completely lost it yet?