for a moment she just stood there,unable to believe what had

状语
再问： to believe what had just happened和unable 有关系吗？
再答： 有关系 It is unable to believe what had just happened 在逻辑上有主谓关系
再问： 那么这句话是省略了it is ?
再答： 这是一种修辞方法，也不能说是省去了it is 只能理解为是用英语如何解释它