“I will have been studying English for 30 minutes when my friends arrive.”

WHEN TO USE IT: The future perfect progressive (also called the future perfect continuous) tense is used to indicate a continuing (continuous) action that will be completed at some specified time in the future.

FORM: [will have been + present participle] or [am/is/are + going to have been + present participle]

Each of the highlighted verbs in the following sentences is in the future perfect progressive tense.

I will have been studying Greek for three years by the end of this term.

In this sentence, the future perfect progressive is used to indicate the ongoing nature of the future act of the studying. The act of studying ("will have been studying") will occur before the upcoming end of term.

By the time the meeting is over, the committee will have been arguing about which candidate to interview for three hours.

Similarly in this sentence, the ongoing nature of a future act ("will have been arguing") is emphasised by the use of the future perfect progressive. The act of sustained arguing will take place before the meeting is over.

When he returns, the wine will have been fermenting for three months.

Here the ongoing action of fermentation will precede ("will have been fermenting") the act of returning.