1、表示在将来某一时刻之前开始，并延续到那个时刻的动作，不再延续下去
The meeting won't be over before lunch time. By then it will have been going on for four hours. 会议将一直开到午饭。到那时，会议将连续进行4个小时。
2、表示动作在将来某一时刻之前开始，并延续到那个时刻，可能继续下去
By the beginning of next month, the play will have been running for two years. 到下个月初，这个剧已连续上演两年。
3、表示说话人对一个已进行的动作的推测或揣想，带有明显的情态意义
You 'll have been wondering all this time how my invention works. 你一定一直都在想知道我的发明进行的情况。
It's early spring. Birds will have been flying back. 现在已是早春，鸟儿该飞回来了。
4、用于条件或让步状语从句中
If you will have been waiting for me at the station for more than half an hour, I shall pay you anything that you ask. 只要你在车站上一直等我半个多小时，你要什么我都给。
Though they will have been wondering where I am, they will not have written to enquire. 虽然他们会一直想知道我的地址，但他们不会写信询问。
