will have done可能是将来完成时，表示在将来某个时间之前已经发生并结束的动作。也可能情态动词的推测性用法，表示说话人站在现在的立场对事件在过去的发生作相当有信心的推测，其信心程度略小于用must have done.。在实际应用中，will have done用作将来完成时远远多于用于表示对过去的推测。究竟如何区分，解析如下：
1. will have done在表示将来完成时时，will是助动词；表示在将来某一时间以前已经完成或一直持续的动作。含义有二：
①表示一个动作到将来某个时间为止已经完成，强调对将来那个时间产生的影响。
Pick me up at 8 o'clock,I will have had breakfast by then.
早上8点钟你来接我,到时我已经吃完早饭了。
You will have reached Shanghai by this time tomorrow.
明天的这个时候你就到上海了。
By the end of the month he will have trained 600 horses/climbed 50 mountains．
到本月底，他驯服的马就有600匹/攀登的山峰就有50座了。
We shall（will） have learned 12 units by the end of this term.
到这个学期末，我们将学完12个单元。
By the time you get home， I will have cleaned the house from top to bottom.
你到家之前我将把房子彻底打扫一遍。
We will have finished the course by the end of the month.
我们将在月底前完成这门课程。
②表示一个动作延续到将来某个时间，并且有可能继续延续下去。
The conference will have lasted a full week by the time it ends.
会议从开始到结束将持续整整一个星期。
They will have been married for 20 years by then.
到那时他们俩结婚就满20年了。
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。
I will have worked at the company for three years soon.
我很快就在这家公司工作三年了。
2. will have done表示推测，此时will是情态动词；表示说话人站在现在的立场对业已完成的动作的相当有把握的推测，这种推测其信心程度略小于用must have done.。
The holiday will have done him the world of good.
假期定会对他大有好处。
I am sure that he will have got the information. 我相信他一定会得到这个信息。
Even if families are less likely to sit down to eat together than was once the case, millions of Britons will none the less have partaken this weekend of one of the nation's great traditions: the Sunday roast.
即使家庭不太可能像以前那样坐下来一起吃饭，但是数百万英国人想必在这个周末参加了英国的一个伟大传统：周日烤肉。（2020考研试题）
By now, a rocket will have set off on its 35-million-mile trip to Mars and scientists must be waiting for the results.（新概念3，L9）
到目前为止，一枚火箭可能已飞越了其3500万英里的火星之旅，科学家们必须等待结果。
You will have received an invitation to the meeting.
你可能已经收到了会议的邀请了。
You’ll have heard of him？
你可能听说过他？
She will have had her dinner by now.
现在想必她已经吃完饭了。
She will have arrived by now.
她现在可能已经到了。
We worked together for ten years. She won’t have forgotten me.
我们在一起工作了十年。她大概不会不记得我。
Can you find her purse? If she’s gone somewhere she will have taken it.
你能找到她的钱包吗？如果她去了什么地方，她可能会带上钱包的。