Future Perfect Progressive

The future perfect progressive is a lot like the simple future perfect, except it talks about a continuous action (and not a simple action) that will have been done by some point in time. For example, the phrase "he will have slept" is simple future perfect; but the phrase "he will have been sleeping" is future perfect progressive. The future perfect progressive is almost always constructed by using the phrase "will have been", followed by the present participle ("-ing" form) of a verb. 

Usage rules

Here is an example of the future perfect progressive being used in a correct way. 

"By the time the morning comes around, he will have been walking for a total of a solid eight hours in his pilgrimage across the desert."

In this sentence, the construction "will have been walking" has been done right. 

Now, here is an example of the incorrect use of the future perfect progressive. 

"We're amazed to think that by the end of this year, we will have been being friends for almost a whole decade." 

In this sentence, "will have been being" is incorrect: this is because the verb "to be" is an exception and cannot be used in the normal future perfect progressive construction. 

In case you are still a little confused, here are a couple rules you can follow in order to make sure you know how to use the future perfect progressive. 

1. The future perfect progressive often expresses the same kind of information as the simple future perfect. The tone, though, tends to be different: you are focusing on the process of what will have been happening over a period of time, rather than just the discrete moment at which that process will have been completed. 

2. In English, the most important exception to the normal construction of the future perfect progressive consists of the verb "to be". You wouldn't for example, say you will have been being somewhere for a year; you would drop "being", and just say "will have been". Aside from this, there really aren't too many exceptions to worry about with the future perfect progressive.  

Future perfect vs. future perfect progressive

It is perhaps worth reflecting a little more closely on the real difference between the simple future perfect and the future perfect progressive. For example: what is the difference between saying that a man will have been walking for two hours, and saying that he will have walked for two hours? The main answer would seem to be: immediacy. Both statements convey the same information. However, the future present progressive takes the reader's imagination to the experience that the man himself will have had over time. Whereas the future perfect expresses a simple fact, the future perfect progressive evokes a sense of process. 

The use of the future perfect progressive can subtly alter the meaning and focus of a given sentence. For example, if you say a man will have walked for two hours, then this seems more like a piece of impersonal information, to be used primarily to check his position on a map. If you say that he will have been walking for two hours, though, you think about the man himself; you think, for example, about whether he may be tired. The future perfect progressive can thus evoke a very specific kind of tone or perspective within a given sentence.