We use Future Perfect Continuous to show that action will continue up until a point in time in the future.
The components for this tense are:
- Action started in the past / present / future
- Action will continue for a period of time in the future
- Action will continue up until a point in time in the future
A point in time in the future can be:
I will have been doing my lessons for 2 hours before my brother comes
We don’t use future tenses after
before, after, when (if “when” is equal to “by the time when”), by the time, if
That is why instead of Future Simple, we used its counterpart from the present tenses – Present Simple (before my brother comes)
However, the first part is in the Future Perfect Continuous: “I will have been doing”
Let’s reverse the same sentence:
After I have been doing my lessons for 2 hours, my brother will come
In the above example, we didn’t use Future Perfect Continuous for the same reasons. We don’t use future tenses after “after.” That is why instead of Future Perfect Continuous, we used its counterpart from the present tenses – Present Perfect Continuous.
time on clock
I will have been waiting for 2 hours by 3 pm.
I will have been reading the book for 2 hours before the exam.
But I started when I was 9, started martial arts when I was 11 and next year I will have been doing martial arts for 50 years.
In three years, he will have been doing this for three-quarters of a century — making his reign one of the longest among theatrical professionals.
contextual time (not explicitly indicated, however, we can understand it from the context)
Come Monday, the war in Iraq will have been going on for 1,000 days and 48 hours later, the people of Iraq go to the polls. A look at the country’s present and its future. (example from cnn.com)
Contextual time: Future Perfect
Contextual time can be any time point (another action, time on clock, event, date) that we can infer from the situation (context).
(question) How will you pass the exam?
(answer) I will have been doing my lessons for a few days! (point in time is not explicitly indicated)
Here, contextual time is “before the exam.” It falls into event category. Just to make sure we are using Future Perfect Continuous correctly, we can convert this sentence:
I will have been doing my lessons for a few days before the exam.
As you can see, the above example circles back to the previous one.
Simple way to explain Future Perfect Continuous
Well, another way to explain Future Perfect Continuous, is not that scientific, however, closer to our human nature, is:
When something is going on for a period of time in the future before something else. The action that is earlier, out of the 2 actions, will be in Future Perfect Continuous.