The peasant learned__many others never understand. that / what...答案為what?為什麼不能用that?

what除了可以當作「疑問代名詞」等外，可以當作「帶先行詞的關係代名詞」，用來引導一個「名詞子句」當受詞。這時what = the thing that。但that沒有這個功能，只能當作「指示代名詞」「關係代名詞」或「從屬連接詞」，當作關係代名詞時，是形容詞子句當作「修飾語」用，前面需要有一個「先行詞」。但本句沒有先行詞，需要使用一個「帶先行詞的關係代名詞」，帶出名詞子句當作動詞learned的受詞，所以應該用what:

The peasant learned what many others never understand. = The peasant learned the thing that many others never understand.

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.