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Future Perfect Continuous has two different forms: "will have been doing " and "be going to have been doing." Unlike Simple Future forms, Future Perfect Continuous forms are usually interchangeable.

FORM Future Perfect Continuous with "Will"

[will have been + present participle]

Examples:

You will have been waiting for more than two hours when her plane finally arrives.
Will you have been waiting for more than two hours when her plane finally arrives?
You will not have been waiting for more than two hours when her plane finally arrives.

FORM Future Perfect Continuous with "Be Going To"

[am/is/are + going to have been + present participle]

Examples:

You are going to have been waiting for more than two hours when her plane finally arrives.
Are you going to have been waiting for more than two hours when her plane finally arrives?
You are not going to have been waiting for more than two hours when her plane finally arrives.

NOTE: It is possible to use either "will" or "be going to" to create the Future Perfect Continuous with little or no difference in meaning.

Complete List of Future Perfect Continuous Forms

USE 1 Duration Before Something in the Future

We use the Future Perfect Continuous to show that something will continue up until a particular event or time in the future. "For five minutes," "for two weeks," and "since Friday" are all durations which can be used with the Future Perfect Continuous. Notice that this is related to the Present Perfect Continuous and the Past Perfect Continuous; however, with Future Perfect Continuous, the duration stops at or before a reference point in the future.

Examples:

They will have been talking for over an hour by the time Thomas arrives.
She is going to have been working at that company for three years when it finally closes.
James will have been teaching at the university for more than a year by the time he leaves for Asia.
How long will you have been studying when you graduate?
We are going to have been driving for over three days straight when we get to Anchorage.
A: When you finish your English course, will you have been living in New Zealand for over a year?
B: No, I will not have been living here that long.

Notice in the examples above that the reference points (marked in italics) are in Simple Present rather than Simple Future. This is because these future events are in time clauses, and you cannot use future tenses in time clauses.

USE 2 Cause of Something in the Future

Using the Future Perfect Continuous before another action in the future is a good way to show cause and effect.

Examples:

Jason will be tired when he gets home because he will have been jogging for over an hour.
Claudia's English will be perfect when she returns to Germany because she is going to have been studying English in the United States for over two years.

Future Continuous vs. Future Perfect Continuous

If you do not include a duration such as "for five minutes," "for two weeks" or "since Friday," many English speakers choose to use the Future Continuous rather than the Future Perfect Continuous. Be careful because this can change the meaning of the sentence. Future Continuous emphasizes interrupted actions, whereas Future Perfect Continuous emphasizes a duration of time before something in the future. Study the examples below to understand the difference.

Examples:

He will be tired because he will be exercising so hard.
This sentence emphasizes that he will be tired because he will be exercising at that exact moment in the future.
He will be tired because he will have been exercising so hard.
This sentence emphasizes that he will be tired because he will have been exercising for a period of time. It is possible that he will still be exercising at that moment OR that he will just have finished.

REMEMBER No Future in Time Clauses

Like all future forms, the Future Perfect Continuous cannot be used in clauses beginning with time expressions such as: when, while, before, after, by the time, as soon as, if, unless, etc. Instead of Future Perfect Continuous, Present Perfect Continuous is used.

Examples:

You won't get a promotion until you will have been working here as long as Tim. Not Correct
You won't get a promotion until you have been working here as long as Tim. Correct

AND REMEMBER Non-Continuous Verbs / Mixed Verbs

It is important to remember that Non-Continuous Verbs cannot be used in any continuous tenses. Also, certain non-continuous meanings for Mixed Verbs cannot be used in continuous tenses. Instead of using Future Perfect Continuous with these verbs, you must use Future Perfect .

Examples:

Ned will have been having his driver's license for over two years. Not Correct
Ned will have had his driver's license for over two years. Correct

ADVERB PLACEMENT

The examples below show the placement for grammar adverbs such as: always, only, never, ever, still, just, etc.

Examples:

You will only have been waiting for a few minutes when her plane arrives.
Will you only have been waiting for a few minutes when her plane arrives?

You are only going to have been waiting for a few minutes when her plane arrives.
Are you only going to have been waiting for a few minutes when her plane arrives?

ACTIVE / PASSIVE

Examples:

The famous artist will have been painting the mural for over six months by the time it is finished. Active
The mural will have been being painted by the famous artist for over six months by the time it is finished. Passive

The famous artist is going to have been painting the mural for over six months by the time it is finished. Active
The mural is going to have been being painted by the famous artist for over six months by the time it is finished. Passive

NOTE: Passive forms of the Future Perfect Continuous are not common.

More About Active / Passive Forms