We think it wrong that he didn't help her.
it是形式賓語,that he didn't help her是真正的賓語.wrong是賓語補足語.正常的語序是We think that he didn't help her wrong.
We think it wrong that he didn't help her.這里it前面不能加that.
再說上面兩句：
I think that it quite good for you 錯誤.這個it不是形式賓語,而是真正的賓語.it后面的good是賓語補足語.it前面不能加that.因為that只能引導從句,而這里不是引導從句.
I think (that)it is quite good for you.正確.it后面加了is,it is quite good for you就變成一個從句了,是賓語從句.引導詞that可以省略.