都没错啦.主要在于你对于句子成分掌握得不够到位.
i think it good for her to work hard.
i find it difficult to do this .这两句话考察的是thinkfind it +形容词 的结构,这里的it是形式宾语.这两句话都不是宾语从句.
而后面的i didn't think it was a good idea for ... 这才是宾语从句,完整的应该是i didn't think (that) it was a good idea for ... 一般宾语从句的引导词that可以省略的.那么从句中it was a good idea for ...it 是充当形式主语~~~