We think it wrong that he didn't help her.
it是形式宾语,that he didn't help her是真正的宾语.wrong是宾语补足语.正常的语序是We think that he didn't help her wrong.
We think it wrong that he didn't help her.这里it前面不能加that.
再说上面两句：
I think that it quite good for you 错误.这个it不是形式宾语,而是真正的宾语.it后面的good是宾语补足语.it前面不能加that.因为that只能引导从句,而这里不是引导从句.
I think (that)it is quite good for you.正确.it后面加了is,it is quite good for you就变成一个从句了,是宾语从句.引导词that可以省略.