the pen __ out of ink,for it __ write答案是must have run,won't请问为什么这里用must绝对的完成时?won't 用一个将来时态这里又是为什么呢

问题描述:

the pen __ out of ink,for it __ write
答案是must have run,won't
请问为什么这里用must绝对的完成时?
won't 用一个将来时态这里又是为什么呢