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：
Maybe by then you will have changed your mind. 也许到时候你已经改变主意了。
“You'll have finished by tomorrow.” “I wish!” “你到明天就完成了。”“但愿如此! ”
I hope I'll have finished the work by the end of the month. 我希望我这个月底能完成工作。
Demand for cars in the developed world will have reached saturation point within 20 years. 不出20年，已发展国家的汽车需求将达到饱和。
Planning so far ahead makes no sense—so many things will have changed by next year. 提前这么久就计划并不明智——到明年，许多事情都会发生变化。
2. 表示动词的持续
即表示某种状况将一直持续到说话人所提及某一将来时间。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 worked for this company for 24 years. 到下星期此刻，我已经为该公司干了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. 星期五以前将肯定会有结果。