我来解释:: what 是要引导从句的 1:what 在从句里做主语: 【what is difficult 】is that 这样的,后面这个is由前面那个is 决定 ,就是要跟他一样。 2:what在从句里做宾语: 【what they say 】is XX 这样的,这个is就由后面的XX决定 如果是XX and XX,那就是are了 本题为第一种情况。
I believe this is kind of agreement for what-clause: what IS difficult to determine IS what ARE difficult to determine ARE no matter what is after the verb.