i wonder what it was (let he down) 为什么错了 ,答案是that let him down

题目

i wonder what it was (let he down) 为什么错了 ,答案是that let him down
i wonder what it was (let he down) 为什么错了 ,答案是that let him down 为什么有what ,that 这是什么结构

答案

lt is⋯that⋯强调句型.
调整下,也就是：lt was what that let him down.
句意：我想知道是什么使他失望.
希望有帮助!