Future Perfect Continuous

How to build it

• Affirmative: will have been + verb-ing: By noon she will have been writing the letter for three hours.
• Negative: won’t have been + verb-ing: He won’t have been waiting long.
• Question: Will + subject + have been + verb-ing: How long will you have been working here by next summer?

When to use it

• The length of an activity up to a point in the future, with for + period and by + time: By June, I’ll have been living in Warsaw for five years.
• Explaining a future (or assumed present) situation by an earlier activity: She’ll be exhausted when she gets here — she’ll have been driving all day.
• It is rare in everyday speech; you mostly meet it when someone stresses how long something will have lasted.

Examples

• In October, we’ll have been working on this project for a year.
• By the time he retires, he’ll have been teaching for forty years.
• They’ll be hungry — they’ll have been walking since morning.
• How long will you have been studying English by the end of the course?

Common mistakes

• Leaving out the duration: the form almost always needs for + a period or a clear reference point. Without it, use a simpler form: I’ll work all day tomorrow.
• State verbs: By May I’ll have been knowing her for a year → By May I’ll have known her for a year.
• Counting results: By Friday I’ll have been writing ten pages → I’ll have written ten pages.
• Word order: will been having or will have be → will have been + verb-ing.

The passive

will + have + been + being + V3 — The letter will have been being written — is grammatical, but avoided in practice: native speakers practically never use it. Say it in the active (By June, they’ll have been building the stadium for two years) or rephrase: By June, work on the stadium will have been going on for two years.