此句中pretend后接一个宾语从句.若是这样
it is very important that you should not pretend______you
know.
则需要用what因为know 后无宾语,语义不完整.不能作宾从.
所给题目中know后有宾语what you don't (know).(它用what理由同上.)
因此第一空不用what而用that引导(此处也可省略)