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，由“will [shall]+have+过去分词”构成：
He will have arrived by now. 他现在应当已经到了。
He will have gone back to England.他想必回英国去了。
When we get there，she’ll have gone to work. 我们到那里时她会已上班去了。
On Monday he’ll have been in Britain for three years. 到星期一，他在英国就满三年了。
