Future in the Past Perfect Continuous

How to build it

• Affirmative: would have been + verb-ing: She said that by noon she would have been writing the letter for three hours.
• Negative: wouldn’t have been + verb-ing.
• Question: rare; usually inside a sentence about the past: I wondered how long they would have been waiting.

When to use it

• Reported speech, when the original had the Future Perfect Continuous: “By June I’ll have been working here for a year,” she said → She said that by June she would have been working there for a year.
• The duration of an activity up to a moment that was in the future from a past point of view: He realised that by the time he retired he would have been teaching for forty years.
• It’s one of the rarest English forms; you’ll mostly see it in written reported speech.

Examples

• She told me that by May she would have been living in Berlin for ten years.
• I knew they would be tired — they would have been driving all night.
• He said that by the end of the trip they would have been travelling for three months.
• We realised that by the time the rescue team arrived, we would have been waiting for six hours.

Common mistakes

• Keeping will have been: She said she will have been working… → She said she would have been working… when the moment is already in the past.
• Using it without a duration: like other Perfect Continuous forms, it needs for + a period or a clear time point; otherwise choose would be + verb-ing or would + verb.
• State verbs: would have been knowing → would have known.

The passive

would + have + been + being + V3 — The letter would have been being written — is grammatical, but avoided in practice. Use an active sentence (She said they would have been building the house for a year by then) or rephrase: She said work on the house would have been going on for a year by then.