before后面是时间状语从句,前面需要一个主句,要有主语和谓语。past不是动词,不能作谓语,要用passed,pass是动词,可以作谓语。另外,before后面的从句中"relized" 应该是 "realized".
修改before不可以使句子通顺。
可以这样:Two days gone before he realized that the tast was beyond his capacity.
Two days had gone until he realized that the test was beyond his capacity.
Two days were past when he realized that the task was beyond his capacity.