将来完成时的用法如下：
1. 表示“将来完成”
即表示到将来某个时间为止势必会完成或预计要完成的动作。如：
When we get there, she'll have gone to work. 我们到那里时她会已上班去了。
I expect you will have changed your mind by tomorrow. 我预料到明天你就会改变主意了。
2. 表示“持续”
即表示某种状况将一直持续到说话人所提及的某一将来时间。如：
We will have been married a year on June 25th. 到6月25日我们俩结婚就满1年了。
By this time next week, I will have been working for this company for 24 years. 到下星期此刻，我就已经为该公司干了24年了。
3. 表示“推测
即表示根据某情况作出的推测。如：
That will have been Roland. He said he'd be back at 7. 准是罗兰。他说他7点钟回来。
There will have been a definite result before Friday. 星期五以前肯定会有结果。
