用一般过去时代替完成时

　　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.