请教各位老师。这是我在《剑桥雅思》范本里遇见的句子。很不理解because后的这个从句，第一，这里没主语，只有一个had，不了解这句话应该如何理解。第二，knowledge后面带了一个been displayed，之前极少遇见这种只用been done的状况。不了解是我语法常识欠缺还是由于句子本身出现了错误。烦请各位老师给予指导。谢谢！
句子如下：
All of this being said, I believe that learning a different language should be not only for satisfying our physical needs, like money, but also moral needs, because never before had such a big thirst for knowledge been displayed.
经查，该文源于《剑桥雅思11》Test 3 Task 2的一篇7分考生作文。作文满分为9分，中间还有7.5，8，8.5三个分数档。因此，7分作文是允许出现一些不影响理解的语法错误的。
这个缘由状语从句 because never before had such a big thirst for knowledge been displayed. 倒装本身没错，但时态(过去完成时）有误。作为议论文的结论，一般要用目前时态，不可以用过去时态。因此此处应改用目前完成时，即had应改为has，即 because never before has such a big thirst for knowledge been displayed.
