I wonder how is Future Perfect Progressive (will have been doing) useful, as I'm feeling that Future Perfect Simple is enough in almost all scenarios. Like

By the end of this year, I will have achieved 500 rep. on this site.

I can't figure out a case where FPP is necessary, or at least makes more sense than FPS.
Also, is there an extreme case where Past Future Perfect Participle makes the best sense?

  • 2
    I think you mean Future Perfect Progressive (not Participle) – SteveES Apr 6 '17 at 14:54
  • They have different emphases. "will achieve" is merely a future prediction, "will have achived" you are expecting the completion or accomplishment, "will have been" emphasizes continuity. – user178049 Apr 6 '17 at 14:55
  • Achievement verbs aren't usually used with the progressive aspect – user178049 Apr 6 '17 at 14:58
  • The difference the perfect aspect can make is not limited to the future tense. Imagine you had an appointment with a CEO and when you arrived at his office, someone told you to wait in the wrong room. Finally, the CEO's secretary found you. She (or he) might say You are here! ("I'd been looking for you everywhere") or You've arrived! ("I thought you hadn't been here"), depending on what she was thinking. – Damkerng T. Apr 6 '17 at 15:13

Perfect tenses present the action with respect to a temporal vantage point.

Present perfect presents it with respect to "now".

Past perfect presents it with respect to a point in time in the past.

Future perfect presents it with respect to a point in time in the future.

In your example, the point in time in the future is "by the end of the year".

The continuous simply adds the idea of ongoingness to the action, presented with respect to that vantage point in the future:

By noon tomorrow, I will have been released from prison, and by 6PM tomorrow, I will have been whistling a happy tune for a full six hours.

By dinner-time tonight, they will have been traveling for eight hours.

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The perfect tenses are not usually necessary, but they are elegant in that they allow you to establish a temporal relationship with few words.

Elizabeth II of England had been only third in the line of succession, and not expected to become queen, before her grandfather died and her uncle subsequently abdicated the throne.

Elizabeth has been queen since 1952, and is presently the longest-serving English monarch.

If Elizabeth continues to reign until 2022 she will have been queen for 70 years.

It's rare that you will ever want to use the future perfect progressive, but (as others have pointed out) it establishes a continuous activity at least until some point in the future.

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