1) 构成will / be going to do sth.
　2) 概念　
　　a. 状态完成：表示某事继续到将来某一时为止一直有的状态。
　　b. 动作完成：表示将来某一时或另一个将来的动作之前，已经完成的动作或一获得的经验。
　　　They will have been married for 20 years by then.
　　　You will have reached Shanghai by this time tomorrow.