The peasant learned----subject + vt
(what) many others never understand---------the object treated as a dependent adj clause,qualifying "peasant".A longer clause is placed at the end of sentence with the purpose to balance the subject of a sentence;i.e.the relative balance of end-object.
For subject + vi----+ (preposition (+ it) clause=(to it)----then "that " is used. eg:-vi=agree ; must see ; decided ; worry ;etc.
Hence for learned =vt.-----then "what" is used.